It's the sharper 'feel' of the spur......vs the dull feel of the boot. We've had horses that you didn't need to ride with spurs and we've had horses you didn't want to ride without spurs. They can make a dull sided horse not so dull.

Spurs are actually designed for 'lifting and lightening' a horse, not to jab into them to make them go faster...but to help them turn/react faster. If you have a dull horse that you have to 'pedal along' a spur will make them wake up and move out better without all that bumping with your bare boot heel. Our daughter had a horse that she ran barrels on.......she warmed him up using spurs and competed without them. If you spurred him to run faster, he'd just hump up and go slower.....but he needed to know that you could spur him.......it's hard to explain.......I don't condone sharp spurs at all, ever. A dull rowel works well.  Some spurs are called 'gut hooks' because of the big pointed rowels.