Mostly, knowledge of a resource, a safe place, where help is available in a non-threatening environment. This can only happen if we get them here once. Poor writers feel even more stigmatized than they did before. Adequate writers avoid what they see as extra work and miss out on a potentially positive learning experience.
Good writers get an unrealistic idea that their writing needs no improvement.
The system is imperfect, students helping students. But our goal, to help student writers acquire new writing skills, requires time and patience. Cranking out error-free documents is, at best, an unrealistic expectation.