Servers which are used in smb businesses dont have enough storage capacity. Reason for this that having large storage devices (harddrives) with redundancy / raids become expensive. Further these drives become a part of the server. Therefore, in order to have more storage space with similar redundancy, NAS is preferred. With nas you can have storage capacities of multiple TBs. These devices can be secured within domains by linking access through active directory (if windows server environment). Further there are UIs which give simple security to share data. SMBs usually use it for document storage, server backups.