No bother Ted and tom - the biggest clue to the monoliths purpose is the intro; the apes start using bones as tools (insofar as weapons are tools), and the connection in imagery between the bone thrown in the air and the spacecraft in the next scene; its not just a nice blend, its saying 'this led to that'. This gives the clue that the monoliths are development accelerators and therefore explains the ending.
I believe the first one, with the apes, was planted there when the aliens visited earth for the second time - the first being before multicellular life arose (obviously that detail isnt in the film, and I'm not certain its in the first book either, but I think it is) and they recognised the potential for intelligence in the apes.
Then the one on the moon is just to check that we've made it that far. It sends a signal to the one round jupiter to activate, and so invites us to follow the signal. Its a series of qualifications; make it this far and you've proved youre ready for the next step. That one takes a bit of adding up from just the film I'll grant you - I didnt see that till I read it, but it is there to be seen/inferred.
Then when they get to jupiter (saturn in the book) the final big monolith makes the last crewman into a non-coporeal super-being (still well less advanced than the aliens are by then though, as the books reveal). Him looking like a fetus is kubric trying to signify all of his new alien nature, his rebirth and the vast amount of advancement still to go with one image.
The biggest clue to how/why HAL went crazy is that he sabotages the com array first - it was earth that was giving him his instructions to lie about the nature of the mission and that was his attempt to sever himself from the thing that was putting him in a position contracictorty to his programming.
Anywho, there are books, 4 of them, and one other (comparatively poor) film based on the second book that explains a lot about the first one. 2001, 2010, 2061, 3001. 3001 reveals by far the most - the fella that drifted off into space is found and revived and the monoliths try to destroy us.