The beauty of these launchers is that you can very easily browse what is available on CurseForge and one click install the pack and then play the pack you have chosen. No need to do complicated things like going to the %appdata% folder, or look for the Minecraft folder, and do this or that to whatever is there. Those days are gone, long gone and if anyone is telling you to do it that old way then that person is setting their selves up for lots of tech support and setting you up for lots of heartache which will probably cause you to leave Minecraft and never look back.
But why should you use a launcher?
Mod packs contain specialized addons that contain local textures, sounds, and animations. These are not assets that a remote server can distribute to a vanilla client. The server has a copy of the pack too, but it doesn’t render sound, animations, or textures, it runs the logic behind the mods. Therefore, playing against a server pack with a local copy of the pack lends itself to a very efficient and pleasant playing experience because now your computer can spend the bulk of its time managing visuals and sound, instead of mundane things like mob pathing and battle mechanics.