And they give root access to the device along with GPL compliance statements, allowing for a flourishing developer community that creates tons of useful programs that can run on the embedded device. See for example https://github.com/reHackable/awesome-reMarkable for a list.
There is also an opkg-based package manager and repo with many packages: https://toltec-dev.org/
There is also an opkg-based package manager and repo with many packages: https://toltec-dev.org/