There are a few alternative e-reading applications. KOReader and Plato are both under active development. KOReader is great at reading PDF documents on e-ink screens, has nicer options for accessing Wikipedia articles, has useful options for tracking reading progress (e.g. you can see which pages you have read, which is useful while jumping around technical books), and generally goes overboard in giving you control over how a book is formatted. I have also seen random applications make their way to the Kobo, and some people use Kobo's for their own programming projects.

Another benefit is the ability to sideload software without jailbreaking it. I'm not going to say it is easy, since you need to know a bit about Unix to package your own software for sideloading (verses something that someone else packaged for you), but at least you can do it using trusted applications (rather than downloading something from a random third-party).

Adding on to what others said - I added a button that can download book files from my Google Drive, and it was as simple as copying over rclone and a one line bash script.

Better reading software. KOReader is fantastic (also on Kindle, if you jailbreak).