Consider a fileserver and SAN environment where you have a number of frontend NFS fileservers and a number of backend SAN disk units, with at least as many backend disk units as fileservers. The obvious sensible way to split the disk units among the fileservers is to have each SAN unit used by only one fileserver, because that makes various things much easier to manage.

(You might even be tempted to design an infrastructure around the assumption that disk units won't be shared.)

The problem with staying this way is that you can wind up with a fileserver that has enough activity to overload its SAN disk units, because you may not know in advance what filesystems will be significantly active. To fix the issue, you need to move some active filesystems to other disk units, rebalancing the IO loads among them.

If you want to avoid user-visible disturbances, you generally can't do this by moving filesystems to another fileserver, but a well designed storage setup will let you do it by migrating the filesystem's storage to another disk unit on its current fileserver. In this case that means getting some free disk space from a less loaded SAN disk unit that normally belongs to another fileserver, since we're assuming that all of the fileserver's proper disk units are overloaded already.

Presto: you have SAN disk units that are shared between multiple fileservers, and your fileservers can potentially wind up spreading across many (or all) of your SAN disk units.