Why is it worth spending time on type theory?

Type theory is to set theory what computable functions are to usual functions. It's a constructive setting for doing mathematics, so it allows to deal carefully with what can or can't be computed/decided (see intensionality vs. extensionality, or the different notions of reduction and conversion in $\lambda$-calculus). Furthermore, just like category theory, it gives a great insight on how certain mathematical objects are nothing but particular cases of a general construction, in a very abstract and powerful way. Look up the propositions-as-sets and the proofs-as-programs paradigms to see what I mean (but there's a lot more than that).

Now, as always there are some cons. I'm thinking of two reasons why type theory hasn't had much success among the general mathematicians:

First, type theory doesn't allow abuse of language. For example, in type theory it is usually the case that if $A$ is a set, then a subset of $A$ is not a set. It is a completely different kind of entity: it will probably be a propositional function, i.e. something which maps elements of $A$ to propositions. Another example: if $n$ is a natural number, then $n$ is not an integer (but something like $\mathsf{int}(n)$ will be). Distinctions like these make some mathematicians uncomfortable, but they prove helpful in dealing with certain things which are maybe more interesting to computer scientists (and of course logicians).

Second, there is no "canonical" type theory. Most of the mathematics done in set theory is actually based on $\mathsf{ZFC}$ (or some extension of it). But the fact that there are many different kinds of type theories makes communication between people harder.

Anyway, there have been many attempts to actually start developing some mathematics in type theories, and some of them have been quite successful: see for instance the work by G. Gonthier with Coq, a proof assistant based on a type theory called the Calculus of Inductive Constructions.

What exactly is (simple) type theory? After reading some historical accounts on the development of logic, I really wanted to know the answer. However, I kept getting lost in the apparent triviality of the (typed) lambda calculus that I got as answer when I tried to look it up. This sad state of affairs was resolved when I came across "The seven virtues of simple type theory" by William M. Farmer. To my delight and surprise, it turned out that "simple type theory" is an elegant formulation of higher-order logic. There is even a simple and elegant proof system for it, which has the same consistency strength as bounded Zermelo set theory (a predicative system also known as Mac Lane set theory). However, the last claimed virtue in that article is Virtue 7 There are practical extensions of STT that can be effectively implemented. The corresponding section 8 Practicality contained an interesting selection of items from Pandora's box, if I may say so.

Let me add some thoughts in response to Pandora's box and Luca Bressan's comment. There exists many variations of type theory, exactly because it is a very practical logical system. However, this leads to many types of (terminology) confusion. Is "simple type theory" the same thing as "simply typed lambda calculus"? To which extend are polymorphism, subtypes and dependent types important? Are they used by homotopy type theory? What about intuitionistic variants? Is (homotopy) type theory constructive, or at least predicative?

I learned to no longer worry about such questions. I'm just happy that I have now enough prerequisites to better understand some articles about the history of logic. In addition, I can now read article and books which expect a certain familiarity with type theory. This includes material related to Haskell, automatic theorem proving, and of course the HoTT book.

Disclaimer: I'm starting to learn type theory, so I'm not an expert.

Personally I've found the following some very good reason to study type theory.

• For start type theory is a different kind of foundational theory, and sometimes working in different background can be interesting.

• As Miha Habič pointed out set theory has lot of idiosyncrasy to make all the classical mathematical construction. Type theory is free from those tricky constructions and all the objects are build in a more natural way: although I have to admit that what's natural is a matter of tastes.

• Type theory has also connections with computability theory: it's a constructive foundational theory and has deep link with computer science, in particular it is a formal system that is also a programming language and so it allows to develop easily proof assistant that can help in verifying correctness of proofs. Doing that in classical set theory can be more difficult, for what I get.

I suppose that there are many other good reason to learn type theory, that I'm not aware for now.

Edit: I've just seen that I haven't addressed the first part, the one about why type theory has so very few question on SE and MO on type theory. Still a guess, but I suppose that is due to two facts:

• type theory, in it's actual form is quite new, and(apparently that's false, see ZhenLin comment below) before of the Homotopy Type Theory's book I've never heard of a reference which introduce type theory in a intuitive way: I suppose that made type theory really a thing for people interested in computation theory and mathematical logic;
• almost all the mathematics had be written in terms of sets, this probably contribute in keeping more people interested in set theory rather the type theory.

Anyway I'm thinking that the trend is gonna change especially for the work of homotopy type theorist, I'm really confident that in a not really far future more mathematicians will use type theory as foundational background (many probably already do it without knowing), so it's just a matter of time, or at least that's what I think.

Hope this helps.