I always struggle to see why people bang on about git bisect. You need tests to make it work. If you have tests, why aren't you running them continuously? If you're running them continuously, why do you need git bisect?
As for the rest of us mere mortals, we sometimes write code that has bugs without discovering those bugs right away. In a complex system, it might not be easy to observe an incorrect behavior and immediately deduce the root cause. "git bisect" allows you to retroactively track down the commit that introduced a behavior change, even if it was something you didn't originally think to test for.
Given that my test suite is generally in the same repo as the code, I'd need to write something that patched my new test into the codebase at every git bisect commit, recompile and run it.
I can see that this may occasionally be useful if you have an extremely hard to find bug, but for me it's pretty rare. In fact I've done this once, ever.
Hence my skepticism when people describe this as "git's killer feature" or whatever other hyperbole.