The main Reason I’ve done it is to install KOreader since it handles other formats, especially PDFs better than the stock Reader. Though you can also run your own homebrew apps as well like other people have pointed out. I personally don’t bother though since Ereaders are usually best for a single purpose, reading ebooks on an eink display, and mediocre at best for other applications. The eink display tends to be its Achilles heel when it comes to anything that isn’t reading ebooks, at least in my opinion.