If I feel I reach a point I can contribute something useful to a community, that is when I create an account. However, I think forcing people to create an account either detours some or nudges others to participate more (whether they have anything useful to contribute or not).
There is your usual suspects: combat spam and the anonymous coward effect but even then there is many ways to combat these without invading my privacy / lessening my security online.
I think it mostly comes down to lazy analytics and shitty metrics. "we have xx registered users",90% of those are made up of shell accounts and inactive users but at least the numbers sound good.
This app thing is just the new incarnation of shitty metrics and the same old story of
developer: oo new tech, me want to makey makey.
boss: can you justify it, isn't our mobile site fine?
boss: ok ok, jesus, fuck. Just, we'll have to make sure users actually install it though, or we'll be out of the job.