I think you're correct that for certain types of apps, this sort of knowledge is pretty useless.

However, once the number of things you're dealing with gets up to, say, the millions, algorithmic complexity can really bite you in the ass and no matter how much hardware you throw at it (rented or otherwise), if the work that a single node needs to do is unreasonably complex, your whole app will be slow for every user, even if you have enough capacity to handle many simultaneous users.

In the mobile space, you can think of algorithmic complexity being a proxy for battery life - if you can make it cheaper to compute, you do less overall work, and the battery lasts longer.