I'm kind of curious... Why was the original Cleric given spells? It seems somewhat redundant, since the Wizard already had the "spells schtick."
Why wasn't he instead just given the ability to heal, or counter negative effects a number of times per day? Or maybe a percentage chance to do so?
What purpose did the design of the cleric originally intend to serve?
I think attributing a
design purpose to the cleric in OD&D is probably overthinking things a bit; I think it was more reaction than action and also based more on the flavor-over-purpose principle.
He's both a holy knight type,
and also be a religious archtype based on Christian priests or saints; it wasn't uncommon for both to be attributed powers common to the early cleric: the power to drive off evil spirits, heal wounds, cleanse areas, and in general defend the populace. He wasn't allowed to use a sword because of the prohibition again shedding blood (I can't remember if this reason was debunked as an urban legend later on or not) for monastic orders. The term 'lay on hands' didn't originate with D&D; healing is the single most common attribute given to
Christian saint-figures (flying or levitation is a close second, along with divination, purifying food, speaking in languages, bilocation, speaking to and commanding animals; some duplicate Biblical miracles such as duplication of food (Create food), bringing water into the desert (create water), Moses' acts (Sticks to Snakes), etc)
Now on the other hand, your archtypical wizard is almost
never a healer. Witches sometimes are portrayed as such, but that doesn't come into play as a
common fantasy archtype until the early Eighties as neoPaganism begins to remake the witch's image. So, someone has to heal people and it falls to the cleric, whose inspirational source already has that rep.