calculating the spectrum of digital modulation methods
Either a sine or a cosine has a 0 at multiple of pi/4. So one or the other is zero (say I is 0, Q is the one you want. This allows you to just read the value; the one that isn't zero (Q here) is that value.

If you read the value at (say) 89 or 91 degrees, the one (I) wouldn't be quite 0. And the one you wish to read (Q) isn't quite its full value. This makes it harder to get the right values. And this reading isn't easier.