It used to be only for transferring funds (probably since the launch of the blockchain.info?), but for the last few weeks/months, I've been facing it at the login stage as well.
Did this start to happen automatically? I just logged in with an old account and there is no need to input my Additional Password when logging in (only transferring funds).
Yes, this has been happening for a while by default (automatically). I didn't enable it anywhere.
As far as I can guess, this has to do something with my dynamic IP (I've a changing IP address all the time, due to which I've to verify my wallet logins every time first through e-mail and then through this secondary password).