Depends on the venue, obviously, and the level of volume required on stage or at the back of the arena. One good reason for having power on tap would be to keep some headroom, avoiding distortion. That wouldn't mean 'loud', though, as the whole point is to not push the amp so hard (a 900w amp cruising along, putting out a 100 or so...).
One could say things have changed since 'back then', but in the '70s, I was using a Hiwatt 200w amp (DR205...) as FOH, driving two pairs of 'fridges'. It was loud enough for most halls and theatres. Bass amps, for 'power' groups, would be 100w into inefficient 4 x 12 cabs, occasionally doubled-up for the really stupidly loud 'Hawkwind' gigs. 900w..? At full tilt that would deafen any folks in the front four rows, and crease their trousers, too.
No, that kind of power is not used for volume in most cases. A decent cab these days will develop nearly 100 db with 1w, 10 db more with 10w, and 10 db more again with 100w. With a1Kw we're getting close to 130 db. Is that loud enough or not..?