Question

why some seats are reserved for SC and ST

Answer 1

because as they are lower casts the government should have to keep some seats reserved for them so that they can also get the equal opportunity as people of higher casts get.
it's just because everyone is equal in eyes of law.

Answer 2

So that voice and needs of people of depressed class can be heard in the Parliament during its proceedings.
If the weaker section are not elected to the parliament and assemblies , then the voice of a significant section of our population will not be heard. That would make our democracy less representative and less democratic.