I can't think of anything that Casual Squishy said that isn't on the money. I'll add one more reason, though...no, make it three:
1)Fall leaves. I wasted at least two hours this past Fall hunting discs that were well thrown and on the fairway, but skipped under leaves, or landed out of sight due to elevation and landed in leaves. So, I walk away from a Champion Viking rather than hunt for it for 20 minutes? Yeah, right... Better still, have technology help me.
2)Ooops, already mentioned elevation.
Playing alone where there is a lot of elevation... 'Nuff said, I hope.
3)Long grass. 300' away and more, can you really see exactly where your disc landed when the grass is fairly long? I can't...