GSTR 3B

Yes it is necessary

but why

If you r not showing purchase from composition dealer, exempt purchases in your return then your transactions of purchases in trading account does not match the gtr Returns. After preparation of final accounts all the purchases whether taxable or nil or exempted or purchased from registered or unregistered everything is taken into accounts. So if you have to account all these transactions in gtr returns, then the annual return in gst site will agree with our books of accounts.