There has been a lot of talk of clerics and their value in a D&D game. This ranges from the old school of whether or not the Cleric is an appropriate trope for a fantasy game to the new school of whether a cleric is needed in a game that also has healing from long rests.
I am firmly in the camp of Clerics are as much a part of D&D as Fighters, Wizards and Thieves.
My first character ever was Father Johan Werper, Cleric of the God of the Sun, Hunter of the Undead. He was a bit of a generic cleric to be honest, and I choose the sun god because I thought that as a quasi-medieval priest the sun would be a major feature of all the is holy, bright and good. Plus I had been reading a bunch of Greek Myths and I thought Apollo would make for a good god. But the real reason I choose the cleric; Turning Undead. That was an AWESOME power in my pre-teen mind. So that has colored my views of the cleric ever since.
(Father Werper, now St. Werper, lives on as an official Saint in COA04: Guidebook to the Duchy of Valnwall.)
In real life I am an atheist, but I like the play the religious character. So clerics, witches, druids, all fascinate me. But clerics are where it all started.
Clerics as Occult Researchers
It is fairly well known that the idea behind clerical undead turning came from Peter Cushing's Van Helsing characters in the various Hammer Dracula films. Why not extend the metaphor to include the rest of Van Helsing's portfolio. As a class that puts a high value on Wisdom then the cleric should be a font of knowledge. Sure, this can also be done by the Magic-User / Wiazard, but the cleric's input should not be understated. Van Helsing is described as a meta-physian or what we might call a poly-math, or man of letters. Wizards, even with schools, don't have the same "Academic" credibility as a cleric can have. Sure the shaman could be considered a cleric in some games, and his schooling could entirely be natural or at least un-scholastic in nature.
In D&D 3, 4 and 5 knowledge of the undead fall within the Knowledge (Religion) or just Religion category. These characters tend to have more training in this area than other characters. While wizards are typically the font of magical knowledge, clerics should be the source of knowledge beyond the ken of mortal man and into the realm of the gods and other forms of the supernatural.
Clerics as the Party Leader
The cleric also can serve the role as the leader. While the cleric can run the gamut of influential high priest to crazy street prophet to diabolic cult leader, players typically take on the role of the cleric of the local church, usually good. Certainly that is what D&D wants you to do and that is fine. This type of cleric also works as the default leader, whether he/she is or not. So if this is the hand you are dealt, then play it because clerics make great leaders. Under most circumstances they access to power, money, a hierarchy and can expect a modicum of respect from the locals. All this adds up to an instant authority figure. Even if they are not.
While this role was stress fairly heavy in D&D4, all other versions of the game also give it tacit, implicit and even explicit lip service. In D&D5 the divine domains of Knowledge and War make for pretty good leader types. Their better saves in Wisdom and Charisma make them less likely to charmed or otherwise controlled magically, so this can be role-played as a stronger than average mental fortitude. Which fits the cleric well.
Cleric as the Party Medic
During my run between 1st and 2nd Ed I created a Healer class. It shared a number of features that my Witch class did including the ability to heal by touch as she went up in level. Completely unneeded in 3.x of course, but in 2nd Ed it was quite a game changer. I also made an NPC healer a pacifist. She would never raise a weapon to any creature unless of course it was undead and then she went all Peter Cushing on them. But running that class and character (she was the only character I ever made for that class) showed me how important the healing aspect was. There was not just the regaining hit points, there was the player morale. Also, since the character was an NPC it was easy not to have her fight, but the Players really did everything they could to protect her.
BTW. Her name was Celene Weper and she was the grand-daughter of Father Werper above. Yes clerics in my world get married and have kids, since it is a life-affirming thing.
Plus keep in mind that Clerics as Healers have a long tradition even in our own world. If ever a character decided to become a pure healing cleric and take an oath of non-violence then I would give them XP for every hitpoint cured and a share of combat XP. I would also give them 2x the starting funds (even though they would give what they don't spend back to the church) to represent the investment their churches/hospitals have made in them. After all, can't send a healer out into the world with shoddy armor. Reflects bad on their organization.
Clerics as Combatants?
It almost seems counter to the above, but clerics are the second best major class when it comes to fighting. Only fighters (and their related classes) are better. The get good saves vs. magic due to their high wisdom, or Will saves for the same reason and their saves are pretty decent to start with. Plus they have one thing fighters don't have, the ability to use magic. "So what" you say, "so can Wizards and even your favorite witch." Yes, but can they do it in field plate armor? Clerics can. Sure they do not get the combat spells the wizard gets, but they have a few good ones too. Creeping Doom is a nasty little spell for Druids. Finger of Death and reversed Heal spells can also ruin someone's day.
In games without Paladins, Clerics are the "righteous fist of (their) god". Wizards don't smite.
Clerics can also be one of the few character types that can actually kill monsters with-out the moral hangups. Even fighters, who get paid, and thieves, that might be working as assassins, don't get the same kind of "get out of jail free card" as do clerics operating within the doctrines of their faith and church. Think back to the Crusades and the Inquisition, the faithful got away with murder, torture and even more horrible crimes in the name of their God and the law had little to say about it or were in collusion with them.
Clerics might then be one of the more well rounded characters in the group.