One benefit of blocking sideloading is that it prevents people who have brief access to your phone (border agents, police, etc.) from installing spyware apps on the phone. On iOS those people can extract your data as local backup, but they won't be able to install any unapproved apps that run in the background and that monitor you. On Android it's trivial to install spyware apps via sideloading and regular users most likely aren't able to detect or uninstall them.