As Right Hand Rule has backed up, and TSR2 has worked out from his own working out of flights, it is purely to avoid confusion.
We could call them whatever we want to be honest - as you should know if you used to file flight plans. But this both helps ATC, and us who have to listen to all radio traffic, but just respond to individual call sign. I might be wrong, but I believe it was at the suggestion of Jet2.com pilots that "Q" suffix was added.
Ever heard of "the cocktail party effect"?
Cocktail Party
If you don't want to believe me, that's absolutely fine! I have no more to say on the matter.
DW.
(edited for my rubbish speeling!)