We use Interia because it takes out the pain of creating that API, and adds a lot of features actually which minimize your code and make you even more productive.
That does not mean you can't use it and create an API for your mobile application, because with Interia you won't really be worrying about your web application, it does not just take the API pain, it solves different things we hate, when deploying for instance, less testing, less issues and configuration, and guess what? even more secure, you will be using sessions instead of tokens, you won't have to worry about CORS and security concerns that come with SPA, and more things, lets say in the future you want to add websockets, I have seen a lot of people wanting to add it to their project lately, you will have to worry only about your mobile application as in Laravel itself it's a small tweaks really specially when everything lives in the same repo, and this is only an example.
So yes for me, if I have the chance to pair Laravel and Vue using Interia I will definitely go with it, and maintain API routes specifically for the mobile application, 0 regrets.