What’s the need for separate network for storage why LAN cannot be used?

What’s the need for separate network for storage why LAN cannot be used?
LAN hardware and operating systems are geared to user traffic, and LANs are tuned for a fast user response to messaging requests.
With a SAN, the storage units can be secured separately from the servers and totally apart from the user network enhancing storage access in data blocks (bulk data transfers), advantageous for server-less backups.