Re: Twinkle, twinkle.
At DAWN and DUSK, with a little bit of overflow into full darkness. Strangely enough, most serious observers don't fire up their instruments until it's really really dark.
Furthermore, the field of view of most astronomical instruments is only a tiny fraction of the Mk1 eyeball, nor are they easily distracted like the human mind. Any given satellite will be in the field of view for seconds or less, and I suspect it would be a trivial exercise for a telescope taking a long exposure to "blink" as a satellite flew past. So even with 20-30,000 satellites swanning around up there, very few visible light observations are going to be ruined by random intruders.
Radio astronomy is a bigger concern since the satellites will be "visible" any time they are in the sky, but even there the impact is small, since by international treaty satellites don't transmit in the bands used for radio astronomy. So again, no great concern.
As for who benefits: all of outback Australia; Siberia; most of Africa; big chunks of South America; Canada; Alaska; international aviation; shipping; AND almost certainly Elon Musk since he wouldn't be doing it if he didn't see a profit in it.