>>96124912
As Basic suggests, reaction rolls are for first encounters with NPCs that you as the GM don't already have a pre-planned reaction for. AKA, if you know these surly bandits are going to try and rob the party, there's no point doing reaction rolls. But if they encounter a merchant on the road, and you haven't got a specific plan for him in the game, then reaction rolls come out. This gives you an idea of how they're meant to be used, to determine what sort of impression people have of the party, based on how immediately agreeable they are. This is why Attractiveness benefits reaction rolls but not Influence skill rolls: being good looking inherently makes people disposed towards you, but it doesn't necessarily make you good at lying or intimidating or etc.

Contests of Influence skills (vs Will or a relevant counter-skill like Body Language/Detect Lies) are for active efforts to try and get something out of someone, and they come AFTER the person's disposition has been decided by reaction. The two don't compete with each other, they complement each other.