The primary reason why SSC/HSC certificate(s) are also insisted upon is for the Proof for the Date-of-Birth.
Legally, the X Class certificate [or rather the School Leaving certificate] is taken as the most authentic proof for the Date of Birth for any person for 2 reasons:
1] Earlier, most often no one used to have a Birth Certificate [though now-a-days this trend is changing].
2] In India, even if someone has a Birth Certificate, most often, the name wouldn't be there in it [since as per Indian culture/practices, a child is named a few days/weeks/months after birth].
Hope this clarifies.
26th July 2012 From India, Hyderabad