Say you have a set of tasks you need to do, and some of those depend on others. Now, you want to perform those tasks in order. The question then becomes: in which order? That's the problem that tsort (1) solves.

It's a way to do some of what make does without having to write a Makefile. 99.9% of the time, it never comes up, but when it does, it is very, very handy to be able to this in a shell pipeline. HTH.