I hope you're looking for kernel engineers in a few years. This sounds like my dream job, sadly I have still a lot of studies left..

sometimes you might not have even printf available (very low-level software comes to mind) so you're limited to using registers. Without a debugger it would be a nightmare.

>Why would you ever need to go below the abstraction level of your high level language to debug.

I don't know what you mean by this. If you don't have f.ex. printf available you have to come up with something else.

>You don't need printf's if you write directly to your screen which is easy to do.

What if the hardware you're debugging doesn't have a screen?

Well, in that case you'll have to make do with whatever interface you are using that doesn't have a screen such as your existing debugger