Until 1949 membership of the Commonwealth required the monarch to be head of state. However at the 1949 Prime Ministers' meeting in London they decided to amend that in order to let India stay. From my quick reading it sounds like India just liked being in it, but as the Commonwealth doesn't really do anything aside from host sporting events, I'm curious why this all happened.
...and has Commonwealth status (generally, not just for India) given its members preferential status for free trade/lower tariffs? Migration?