There are a (very) small number of people who jailbreak and use their Kindle as a general computing system. I see no problem with that, assuming you know why you bought it and what you want to do with it.
For a more generally applicable reason, how is this for thought -- many of our jailbreak add-ons greatly enhance reading on a Kindle.
Some people like the Kindle ecosystem, but also would like font flexibility. They are a valid userbase.
Some people want to manage collections.
Some people want to enhance the margins.
Some people want to have nice screensavers -- I personally use the front cover of whichever book is open.
Some people want to protect their privacy, whatever that means.
If it makes them happy...
Some people want a better PDF reader on their E-Ink device, for when they read the odd PDF.
The list goes on and on.
@cvkemp -- Forgive me, but the question seems rather frivolous. All you have really established is that you personally don't get any value-added benefits, and you mentioned in your own post the idea of screensavers and fonts. You have answered your own question, within your question...
None of these things would exist if there wasn't
someone who found it beneficial. Thus, the mere fact that a jailbreak exists is
proof positive that there is a reason.
Now, if your question was: "is there any reason for me, personally, to jailbreak?" I would say, no, not really. If you are happy with your Kindle as is, then that is perfectly OK.