I have perfect pitch and it's overrated.

Pro tip: You can pick any note you want as the starting pitch, and do realtive pitch from there. It doesn't matter. Pitch is an affine space, there's no 0 pitch!

Classic Joke: How do you know a person has perfect pitch?

I learned that as you get older, if you don't use it, you lose it.

Oh of course, if you just want to practice sight reading on your own, correct pitch is irrelevant, but if you want to use an app like this and have it follow along and check if you’re accurate with your sight reading, you have to agree with the app on pitch!

Can you change the tempo or something to slide around on the first note until you find the pitch it wants?

In music school, I was in the position of having excellent relative pitch and zero perfect pitch. In the ear training courses, I actually scored better than the peers with perfect pitch, as it seemed they had difficulty with a lot of the exercises, particularly the by-ear transcription of the four-part bach chorale snippets.

It becomes important if you play with other people though

Even then, you need to tune against those other people. Perfect pitch is not so precise that everyone can just start on exactly the same note hah.