Michael Mitzenmacher discusses the sub aims of PhD students. His main point is that a PhD is a long term process, and if one does some parallel processing one can achieve more targets than simply finishing a PhD:
Graduate school is a long stretch of time — 5 years (or more) for most people. There are few clear goals during that time, although the obvious one is to learn how to do good research, with the hope of getting a tenure-track faculty job. With this somewhat singular — and difficult — goal, it’s easy to fall into the extreme of focusing only on your research to the exclusion of most everything else, or to waste a lot of time not really working. …I thought it worthwhile to suggest some of the additional skills one should try to develop in graduate school over those stretches where you need to break from research — skills which, unfortunately and understandably, are often given short shrift by the university.
Mitzenmacher is of the view that a PhD is a good time to improve time management, writing, speaking (giving talks), leadership (mentoring undergrads or leading a small team) and entrepreneurship. This is a sensible idea and it is always good to pick up a new skill or a language. He also points out that one can and should learn complementary skills:
If you’re a theorist, learn to program a little. If you’re a systems person, learn some probability or other theory. Maybe set aside a few days to learn time-saving Latex tricks, or some other piece of useful software. There are plenty of skills that will make you a better researcher/teacher/writer in the future.