Someone could "use your code and not give back" while staying perfectly within the limits of the GPL, though. For example, they could use and modify your software in an internal package.
The important point is to prevent freeloaders from taking credit from your work [and sometime even competing against you].
However where 'personal use' ends and 'publishing copyrighted code' begins is not entirely clear to me. Does an internal tool in a company still count?
I would think so, yes. If I take a piece of information and do something to it inside my house, that's my business. If I show it to my friend, it's between him and me. If it let my coworkers see it so that they can only see it within the office, it's between them and me. And so on. The only issue would arise if an employee asked to see the source of such an internal tool and was denied.
It's probably a good thing on balance since it's one less barrier to companies using code under the license, but considering companies and their assets (including their internal tools) can be bought and sold, I'd personally consider that distribution enough.
This seems different from something that's purely personal use. I don't think it makes much sense to force a program to show a notice its GPL licensed if you're the only one using it for instance. In fact that seems to run counter to the intent of free software.
It's possible that legal systems treat this situation differently, but I think it makes far more sense if you don't need an additional license to use stuff that's already published by the copyright holder until what you're doing starts to go beyond just personal use.