it doesnt really mitigate damage that much. if you watch earlier in the clip he does it for brambleback and still gets hit. more than anything i think it's just a habit, but the one major benefit is that once you get into a rhythm of how fast you can hit then you can start clicking to attack while moving. the reason why jarvan didnt receive damage from the raptor camp is because he was healing with a potion. auto attacks hit regardless of range once they've reached a certain point in the animation. it might slow down attack speed on melee champs, but raptors are ranged either way. it's referred to as kiting i think and the idea is that you move away from your opponent and cause them to start their attack animation then reset it while moving back into range and attacking then leaving range rinse and repeat. the problem with jungle camp kiting is that you cant ever leave their attack range because they'll reset aggro and heal up completely. of the 6 camps 4 are melee (golem, bramble, wolves, and krugs.) so if your movement speed, range, and attack speed are high you can mitigate some damage, but not much.