Graduate student in medieval literature, here - so possibly the most seemingly impractical career choice ever, especially given my competencies in physics/math, but here are some of the main reasons:

1. Research, research, research - and the appeal of my own specific field, Anglo-Saxon/Old Norse literature & linguistics

2. Teaching - I do in fact enjoy lecturing and leading discussions. A great deal of theatre training has translated into ease and competence when speaking to large groups.

3. Relative flexibility and freedom - when working as a TA and a substitute lecturer, I have loved the freedom to set my own schedule (for marking/prep/etc.), outside of specific class times.

4. While in grad school, and afterwards if I manage to get an academic job: guaranteed support while I spend my time reading and writing and learning, which is what I'd want to do anyways.

5. Environment - I love the university environment, because it allows me to connect with (and learn from) researchers in so many different fields, not only my own.

6. I haven't been as concerned about effective altruism as many here, but helping students develop critical thinking, writing, analysis, and communication skills is definitely important to me.

7. I believe, based on my experience thus far, that I do have the ability to be competitive on the academic job market.

Other related career options that I will be prepared for if academia doesn't pan out: editing, publishing, writing, journalism, library science, translation, teaching ESL. I also work in professional theatre (as an actress & stage manager).

(Convenient timing - I just posted, yesterday, some of my thoughts on this to my blog. Feel free to read if interested: http://merelyinquisitive.com/2014/03/10/why-graduate-school/ )