I quite like the World of Warcraft: Legion companion app; it gives me a chance to send off my companions on missions, start research, and check on world quests whilst I’m out. All of these things are convenient (and because of how the game is set up, they’re also not self-sustaining; I need to log in and play to earn resources to run those missions). But they’re also all just ways of automating stuff that’s already in the game. There’s nothing unique that I can only do if I have the companion app, which was a trick RIFT pulled that I rather liked.
At the same time, I can also understand players not wanting that; who wants to turn playing the game to involve your phone and your computer and other devices? Probably not many people. But that just brings to mind the obvious question of what people really want from these apps. Do you want to be able to handle your auctions and chat remotely? Access content that would otherwise be inaccessible? Take care of minor chores that can have odd cooldowns? Look up information about your game of choice in a database? What do you want companion apps for MMOs to do? Or would you prefer they didn’t exist at all?