If you set RequireSecuritySignature=1 on servers, the registry setting ensures that the Server communicates with only those clients that are support message signing. BEWARE: older clients will fail to connect to servers that have this key configured. Similarly, the clients with RequireSecuritySignature set will not be able to connect to servers which do not have message signing support. A little looser but more reasonable approach is to set RequireSecuritySignature=0 and EnableSecuritySignature=1. Then if both ends of the converstation have been configured for SMB Signing, it will work and if one or the other is not configured, communication can still occur. Setting RequireSecuritySignature=1 on either the server or workstation is for environments with quite sensitive data as a rule.
The need for SMB signing has become less theoretical with the release of the hacker tool SmbRelay which automates a man-in-the-middle attack against the SMB protocol.