There are plenty of mods that are addons to another mod. Cobblemon is a big example, but also a bunch of others. When I searched modrinth there was at least 1000 mods with it in the title. Besides being a useful indicator in general, it could also allow people to set the parent of their addon to the mod it's an add-on to to help clean up mods list (Useful for cobblemon, where you can easily have a bunch, or Limelight, were a person could have a bunch of extensions added)

Actually, this feature is included. Addon author can providing mod parent info in their modmenu info in fabric.mod.json.
For example, see https://github.com/sakura-ryoko/litematica-printer/blob/316620cc26db6f17e9cc8d3b8ee1432195e819dc/src/main/resources/fabric.mod.json#L34 (litematica printer is a extension for litematica)

Sorry, I should have been more clear, I know the parent can be set, but without a badge, it can look like it's a component mod (like fapi modules) rather than a 3rd party mod one