Well... first I thought that the answer would be really simple but... (lucky me)... it wasn't so I had to go around and look for the information through about 8 books! That wasn't fun! You'd think that it was built there because that was a place in Egypt where they buried the gods and had the Sphinxes there to protect them, but that's not right. The location of where the Sphinx would be built was very thought through and carefully chosen. Seems kinda spoiled to me... I mean why would someone have to go through all that work to find a spot where to start building the pyramid and the sphinx? It's really simple... find the source of the rocks and then build there! That way you don't have to move the blocks so far!
The Sphinx looks east and seems to guard Khafra's pyramid of where he was buried. Sphinxes jobs were to guard temple gates so of important kings/pharaohs they would have sphinxes there to guard them. While it might seem like a common thing to have a sphinx guard a king in the after life, queens were not given the same opportunity as often (which I don't understand because how come the let women become pharaohs or Queens but then they can't have the same rights as the Kings or men?!). They were very rarely seen near the pyramids of the women leaders... still... I don't think that's right... We fight for the rights of women now and back then women were thought of to be very powerful, so... back then they should of had the same rights too!