It also helps build a relationship. At some point that recruiter might 'ask' you to take a job with his/her client. Saying no to someone you haven't met is easier than saying no to someone you have spent time with.
Recruiters are constantly doing little sales 'closes' during the process, building towards that big close of you accepting a job offer. It's all part of a sales process, no different than any other sales position.
Edit: changed "the reason" to "one reason"