The Call of Duty: Advanced Warfare Companion app does two things. Lets you edit emblem and communicate with your clan. You can also look at your multiplayer stats, but there aren't any options to customize your loadout.
Essentially if you don't participate in clans, there is no reason to download the app because it will just take up space but not offer any additional purposes. If you are part of a clan, then it's a great way to keep up to date with your clan.
This app is only meant for clans and no one else.