IMO, the greatest benefit of graduate school for me was learning how to learn. I don't mean this in the cliche "knowledge for knowledge sake" way, but that I learned how to go from zero to expert in a given subject in a very short time.
I attended a BPU (big prestigious university) studying Mechanical Engineering. I originally came to grad school intending to do work in the heat transfer field, but got an opportunity to work in a supersonic wind tunnel (i.e. big freaking hardware) and switched to fluid dynamics. I had only moderate exposure to compressible fluids in undergrad, so I was forced to bone up in a hurry. When my experimental setup required high-speed data acquisition, I learned how to do that. When the analysis branched off into chaos theory, I learned that. My mentors were helpful, but mostly I learned by seeking the right references and reading up on the subject at hand. This was all pre-web, so I spent a lot of nights in the library.
When I finished the PhD., I took a postdoc position studying low-speed laminar water flows. This is fundamentally different than the supersonic work I'd been doing, so it was back to the library. Of course, I'd covered the topic in textbooks, but it takes a lot of time to look at the actual research papers being published in a given field and find out what's been done and what the state of the art is (much less learning the names of all the "players").
When the postdoc wrapped up, the web had just been born and I left "Science" for an internet startup. In due course, I learned HTML, Perl, Java, etc. In each case, the regimen I'd gone through in grad school is what allowed me to focus properly and learn quickly.
Caveat: Obviously, you don't have to go to grad school to learn these skills, and anyone who's self-taught in any computer skill has the means to self-teach again.
Aside: Actually, the main focus of my time in grad school was club-hopping nightlife. This in no way contributed to the "learning" aspects of this post, but it's either a good reason to go to grad school or a good reason not to, depending on whether you regard living life as a waste of time. Now that I've moved on to running a business and a family, I can look back at grad school as the last time I had absolutely no external contraints on my time. I don't miss it, but I do cherish the memories. IMO, grad school provides you a relatively unstructured situation in which to explore both academically and non-academically. That in itself is an opportunity.