Anecdotally, people tend to design for things to break at distinct breakpoints. Part of the reasoning for that is that they don't want things 'jumping around' at different viewport sizes.
My problem with that: I don't think users actually resize their browser much. Sure, we have to design for many different viewport sizes, but adjusting at specific, shared breakpoints doesn't help with that. In fact it can even work against that, as breakpoint + 1px often looks poor compared to the breakpoint itself, where everything shifts.
Even if they are resizing their browser, they're going from A -> B, not A -> A1 -> A2 -> A3...
So the problem is two-fold:
This is why [[intrinsicdesign]] likes to move away from the idea of breakpoints