If it's actually a good reward. But most of the time it's really not. I use to do optional stuff in games. But Zelda OOT and MM taught me side missions are time wasters. Just not really worth it. Even games with something that suppose to make things easier. Like this rare item has better stats. Only to find out it breaks faster. So it's really not better. The issue is diverted to another area.
The game that got me into doing this was Final Fantasy VI. You get two swords from optional bosses that let you attack 12 times with a single character. From then on it was this feeling of needing to do it so I don't miss out on anything. That and Tales of Symphonia that had the giant skeleton in the opening and I was like "Yeah, I gotta beat him".