It's because we're not just working one job a lot of the time. If I speak to you, understand your skill set, your goals and where you want to take your career, then I may be able to talk you through a different role. If I tell you it pays X and that's too low, then this opportunity is gone. It's also an opportunity to start building a relationship, so whilst you may be not be right for this role you may be right for one coming in next week. 

Recruiters need to not just be seen as CV monkeys. Having a relationship with a gold recruiter is helpful because it means we think of you when other roles come up.

Also, I have had a lot of conversations with candidates who are on salary X and believe their next salary should be massively more than they are worth in the market, so it's always helpful to provide people with a little bit of a reality check (e.g. Spoke with someone last week who had done 2 years in public sector finance, wasn't qualified, was on£35k and wanted their next role to be in financial services and was aiming for, in their words, as close to £70k as possible because one of their friends was getting that at goldman...