Source: Adaptec

A single SAS link to an expander that hosts multiple drives would, of course, be an obvious bottleneck, so the specification provides for wide SAS ports. A wide port groups several SAS links into one interlink between any two SAS devices (usually a host controller and an expander). Links to attached expanders can be scaled up as your storage requirements grow. However, redundant connects are not allowed as this is taken care of by wide links; any other sort of loops or rings are not supported, either.

Source: Adaptec

Future SAS implementations will provide 600 and 1,200 MB/s bandwidth per port. As hard drive performance is not going to increase that much, it becomes obvious that using expanders on a limited amount of ports seems to be a good approach.

Devices called "Fan Out" and "Edge" both are expanders. However, only one Fan Out (primary) expander is allowed for a SAS domain (see 4X link to initiator in the middle of the image). There may be up to 128 physical links per edge expander - whether these are merged into wide links and/or linked to other expanders or drives. The result can be pretty complex, but equally powerful and flexible. Source: Adaptec.