In the context of git bisect, it's vital. You can't go looking for bugs commit by commit with no requirement that any individual commit is actually buildable or functional.

In my own projects I'm as guilty of this as anyone, because it's nigh impossible to get the tools to actually do this (which has a massive effect on the culture which builds up around them).

If you know which commit introduced a bug you know which exact lines to look at - it makes understanding the reason for a bug much easier.

If you're good about small commits it reduces the lines you need to look at to less than 100 usually.