In India some laws are there that are to be applied to Muslims,christians and people of other religion differently.Laws relating to marriage ,adoption,minority etc. are not necessarily the same for parcis,muslims,christians or any other religion.For example a hindu cannot practice practise polygamy but a musalaman can marry more than one wife.
So for such legal purposes registration specially when converting ,if you want to be known as a person from for examlpe hindu religion legally, is required.
jai ram