One of the main reasons why I find the theory of computation (“my” branch of theoretical computer science) fascinating and worth studying is the following one: it provides us with a way to investigate some deep (and sometimes puzzling) philosophical questions.
One of the founders of the theory of computation, Alan Turing, tried to pin down the meaning of “computing a function” for a human being equipped with a piece of paper, by giving a mathematical description of the process. I’m not the only one to think he was extremely successful, and Turing machines proved to be an accurate model of many other computing processes.
Now that we possess a class of mathematical objects describing computations, we can actually prove theorems about them, thus trying to uncover what can be computed, and how it can be computed; it immediately turned out that lots of perfectly legitimate functions cannot be computed at all, and that they can be classified according to a degree of uncomputability (some functions are just “more uncomputable” than others).
Some other guys, the first ones usually identified with Juris Hartmanis and Richard E. Stearns, tried to describe mathematically what it means for a function (resp., a problem) to be hard or easy to compute (resp., to solve). There are several complexity measures according to which the hardness of problems can be described; the most common one is just how much time we need to solve them. Alan Cobham and Jack Edmonds were quite successful in identifying a reasonable notion of “efficient computation”.
Within the computational complexity framework, we can now prove some results that are consistent with our intuitive notion of computation. My favourite example is the time hierarchy theorem: if we are given more time to compute, we can solve harder problems.
The central open problem of complexity theory, P vs NP, is just a formalisation of another philosophically significant question: is it really harder to solve a problem than to check if an alleged solution of it is indeed correct? I believe this question is worth asking, and answering, independently of its practical significance.