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One reason would be to a get a newer version than what your distro offers.

Some version of LibreOffice is native to a distribution. Those new formats have benefits, including the ability to automate the processes of updates (mostly thanks to better tooling as they are newer), the ability for confinement, etc.

> including the ability to automate the processes of updates

... What exactly will it do that's better than being included in `apt upgrade`? I do not want to go back to the Windows way where every stupid application insists on running its own little auto-updater in the background.

You can run multiple different versions alongside each other, and you are not restricted to the version that your distribution happens to carry which is often outdated. Also, this gives you access to development snapshots