Asking what photography is "for" may be pertinent -- I don't think it is ... but that still begs the question of what is the most important single thing in photography.
To communicate clearly - the essence of my long post.
Aunt Emma snapping little Sam's birthday party is taking photographs to tell you something - to communicate. The message might be obvious and deliberate ("Sorry you couldn't be here but the kids had a fun day - look!") or something subtle that Aunt Emma isn't thinking of consciously ("It's sad I can't have children") - but regardless of whether the photographer intends to convey a message, all photographs communicate.
Case study 2
Man Ray's "Object to be be Destroyed":
(Image copyright: The Museum of Modern Art, New York)
In 1923, Man Ray created this, then untitled, playful art object, a metronome with a woman's eye, as a silent witness to watch him work. However, in 1932, the object was remade, and had a title, "Object of Destruction", and instructions:
"Cut out the eye from a photograph of one who has been loved but is seen no more. Attach the eye to the pendulum of a metronome and regulate the weight to suit the tempo desired. Keep going to the limit of endurance. With a hammer well-aimed, try to destroy the whole at a single blow."
The eye that Man Ray used in this new version was that of his former lover, Lee Miller, who had just left him.
The message conveyed by the use of an eye from a photograph of Lee Miller is obvious and very deliberate - and aimed at an audience of precisely one. This art object also shows the importance of context when displaying a photograph: the original portrait of a venerated lover has become something sinister.
The most important single thing in photography is to communicate clearly...