When one makes their way into the Nights Watch, they arent simply made a Sworn Brother immediately. While the organization has traditionally lax rules for recruitment, there is still a period where those that sign up are simply conscripts, being used where necessary to keep those south of The Wall safe. Eventually, if they survive, and with proper training, they will have the opportunity to join the Nights Watch proper.
The Conscripts Unit Box for the Nights Watch army is not as strong or as tough as many of the other units available to Nights Watch Commanders. However, what they lack in abilities they make up for in ease of fielding. Nights Watch units can get costly points-wise to bring to battle, so the Conscripts can help balance numbers closer to equal. And the Watch Recruiter Unit Attachment can help any unit in the Nights Watch army work just a bit more efficiently.