1) NDA may stop someone from disclosing information.... it may not, but it may. That has a degree of value and the document is generally pretty short and simple to understand so not too problematic.

2) for the same reason many people will not sign an NDA (trust, hassle) you may want them to- trust, commitment.

3) even if an NDA only serves to regulate "the aftermath", that too can have value, especially in the rare but possible cases of mis-use of information e.g. by a competitor...