Don't think anyone else has said, but apart from the possible symbolic aspects etc Harv mentioned, it's often held to have been a kind of 'landmark' for people processing along the Ridgeway towards the Avebury complex.
The 'top of Silbury seen along the Ridgeway' is a bit like Magret Curtis's 'Man standing in the moon' ideas. . sort of interesting, but at best a theory. I just has me going . . riiiiight!
Still, it's a lot better than the 'flying saucer landing pad' I suggested isn't it? Every-one knows UFOs prefer to land in cornfields :-)
'specially in Wessex!