Since they don't seem to know what SO is, I'd say start with that.
Simply put, StackOverflow, specifically (though its other objective sisters, such as Serverfault fall under this, too), has questions and answers that are objective, and therefore, provable. Either the proposed solution works, or it doesn't.
It's targeted. StackOverflow is specifically designed for programmers to help each other. Serverfault is specifically for server admins, and so on. Therefore, it's more likely to attract people that are well-known as experts in the field (for example, Phil Sturgeon, a big contributor in the CodeIgniter community, is an active SO member) than, say, Yahoo answers. If you ask a question on SO, there's a very high chance of it getting seen and answered by the high-profile, heavy hitters in that technology. Who better to ask for help on something than the creators of the technology?
It can be a passive way of finding answers. Generally, when I ask a question on StackOverflow, it's after I've exhausted my mental pool of Google search terms (which often lead to SO questions anyway, I'm still not sure how any programmer hasn't heard of this place anymore, but that's beside the point) and my own ideas for solutions. So, once I ask a question, I move on to other problems, so I don't get stuck in "forest for the trees" mode on that one, and wait for answers to come along. In that sense, I'm more productive, because I'm not spending more time re-searching and re-digging through Google for an answer that may or may not exist in writing yet. Once someone proposes a solution (and they're generally quick), I can do the legwork of getting it in and adapting it to my specific needs.
It helps the programmer community. If you fully participate in SO (ie - you accept answers, vote on questions and answers, and submit your own answers), then you're helping any other poor sap that might come along after stumbling over the issue you once had, yourself (after all, if you had an issue, someone else is bound to have had the same, or something close enough to apply). At the very least it gets more info out there. Even if you never hear feedback from these people, remember the "silent majority" that come across these resources, but don't make themselves known, even if you did help them.