One of the reasons that we use SAN is to make it appear to the server as if it were local. What is the advantage of a SAN over a NAS storage that is mounted to a device?
A SAN appears as a raw block device and can do the things that a block device can but a filesystem can't. Two very common cases include data storage where an application is using the block device directly (as some databases can), or to serve as a boot volume for a diskless server (some servers and blade systems can speak iSCSI from firmware well enough to treat a SAN device as if it were locally attached long enough for the OS to get up and running).
A NAS is a (possibly) simple storage device, featuring various protocols for storage access, often including iSCSI. There is an extremely wide range of different devices, features and performance grades. Most NASes provide disk redundancy, some power and network, few controller redundancy.
A SAN is a network between storage devices and hosts, providing and securing access to NFS or block storage. Most often, it is redundant and designed to scale significantly (many hosts and storage devices).
A SAN can be as simple as a single switch but at least two switches are required for (path) redundancy. In extreme, a SAN can encompass hundreds or even thousands of ports. The most popular SAN protocols are Fibre Channel, iSCSI, and NFS (not necessarily in that order), each with its own pros and cons.
Most SANs use storage arrays that aggregate numbers of hard disks or SSDs. That could be a NAS but most often more sophisticated devices are used with a high level of redundancy - disks, network, power, cache, controllers. Business-grade arrays realize more functions in hardware than a NAS and usually perform faster. Most often you've also got an SLA with the vendor to provide support and fast replacement of parts.
Advantages of SAN storage arrays include performance, scalability, availability, and feature level (snapshots, hierarchical tiering, backend mirroring, ...)
Well, first of all, NAS is a single point of failure which means that if your NAS device dies you lose access to your storage. However, with SAN (Storage Area Network), you have multiple devices acting as storage. Therefore it eliminates that single point of failure and makes your system redundant.
Another advantage of SAN is its speed which is achievable via Fibre Optics and FC(Fibre Channel) Protocol. There is also a cheaper alternative such as ISCSI that works on top of TCP.