Why does home manager also have an option to install user packages?

nah they basically do the same, the home-manager nixos module implementation actually just passes it to users.users.<name>.packages but as mentioned in the comment above it’s quite nice to have the option if you use home-manager on non nixos machines and also be able to add scripts and stuff in to the users path from inside the home-manager config, I for example use that hear: Home-Manager toggle between themes