You need to define your purpose for even having a box set before you start creating things without any strategy.
Timely. I am gearing up to release book 3 in an open-ended series, and my default assumption was, "Time to make box set numero uno," if only because apparently that's what everyone else does. I'm very glad you posted this, as I now have a better grasp of the *why* of it, not simply "that's just how it's done." I'm learning slowly not to do things "just" because other authors do it. Joy of diving in, then learning to swim!
I also see authors making box sets out of their books as soon as they can, mostly KU authors who seem to be chasing the low-hanging page reads by pricing a box set at 99c. If they can get into bonus territory by doing that, I can see the reasoning, but if not, I think that's burning capital. I think a lot of them do it because they get desperate and panicky to keep their momentum going.
The purpose of a cheap box set should be primarily to gain new fans, IMO, which means there needs to be something at retail price for the new fans to read, KU aside, otherwise some of the effect is wasted. Unlike with, say, beer, you can usually only sell a book once, so you don't want to undersell it. Plus, all the "perceived value" reasons for the long term.
Releasing a box set during a lull in your publishing, say when you've finished a series and are writing the first three of the next (intending to do a rapid launch), is good strategy IMO. It keeps a release in the public eye, and can give a shot in the arm to the last series before the next series brings you renewed discoverability.