Is sci-hub also used in places having institutional access?
The only study that I'm aware of is the magazine article that Orion linked to in the comments. Anecdotally, however, my experience matches yours -- even in places with institutional access, some people prefer sci-hub as it's quicker and easier. No need for a lab-specific login or password, no need to check if the journal is actually one that your institution subscribes to; just paste in the DOI and go. In fact, there are even web browser extensions that automatically append the sci-hub URL to the page you're visiting, so with a few keypresses you can be automatically forwarded to sci-hub's cache of that page.