Since this application does everything for you, from localizing the modpack on the Internet, to downloading it on your PC, installing it in your Minecraft folder and launching the host game at the end, it will need you to provide it with your Minecraft credentials. However, you should know that there are dedicated applications such as Technic Launcher that gather several modpacks in the same place so that you can access them with ease. These packs are conveniently called modpacks and can be found scattered all around the Internet.
If you like playing Minecraft on your computer, you are probably familiar, and if not you might've at least heard about how you can enhance your playtime experience by modding the game with various packs.