makes it easy for players to install your mod pack so lower support burden for you
your pack might be more sticky, if someone already has your launcher installed they might be more likely to play again instead of bothering with installing something else
require specific client mods. you might have mods that arent actually required to connect to your server but all players should use to keep a level playing field. for example, the use of client mods that give a disadvantage like Total Darkness could be enforced.
automatically update your modpack
build a sense of community. have links to your projects discord, youtube, etc and display news about your work in the launcher that users might not see otherwise.