The “right” reasons for writing are many: the satisfaction of digging deep into a topic and seeing a long capstone project through; proving to a PhD admissions committee you “have what it takes” to become a professional academic; taking that last opportunity before finding a job to explore something you’re really passionate about; and yes, even just to get the highest level of honors you can.
However, there are other ways to make your last year at Harvard memorable, and these may even be a better use of your time. The point is, only you can determine if writing a thesis is “right” for you. You shouldn’t write a thesis because you think you need to do so in order to get into law school or business school (you don’t), or because someone told you it is what everyone does (not true). If you do write a thesis, it should be because this is something you want to do for yourself. And if you decide that instead of writing a thesis you’d rather take great class you never had time for, devote more time to an extra-curricular activity, or even study for your LSATs or GMATs—that’s okay too.