It is true that there are many free text editors out there. Sublime is not an IDE, and so one might think "Why should I buy it?". That and there is no time limit to using the trial version. I suggest you take a look at http://net.tutsplus.com/articles/news/perfect-workflow-in-sublime-text-free-course/. Mr. Jeffery Way shows a lot of features that are unique to Sublime Text. I've watched the first 10 or so and I think it is very cool compared to other editors.
To answer the "Why should I buy it?" question:
It depends on whether you can afford to pay for it. If you can't, you'll have to put up with the unregistered version's reminder pop-up when it shows up. Think of it this way though: let's say you made a really cool program or website that offers very useful services. Depending on how many people are helping you (if any) develop the project, as well as how time consuming it will be to implement the project, you will probably want to get some compensation from others using your services. For example, if I make a simple tic-tac-toe web-based game, I probably would not charge people for playing the game. If I make an application that helps businesses run more smoothly, such as a mobile app template that allows customization with pre-built functionality (add clients, calculate expenses and earnings, etc.), it would probably take me quite a bit of time and money as well on my end (I may need to hire other developers more experienced in a particular area, they will want to get something too.) It is reasonable to charge some money for these services; I may give a free month to new users so they can see if they like it, and then charge a reasonable monthly fee.
Mr. Jon Skinner is being very nice to us and only charging a one-time fee of $70. You are free to use this license on as many computers as you want. For the amazing features unique to Sublime Text, as well as the license terms, I think it's very affordable and a nice way to help Mr. Skinner out; I'm sure he has expenses of his own to cover.