SCSI as we knew and loved it is slowly going away in favor of SAS. SAS, unlike SCSI, is more Fibre Channel like in that more than two servers can talk to the same devices. Because of this, if you have a large SAS-based disk-pack, you can share that disk to a bunch of servers in the same rack without having to bust out the Fibre Channel cards.
But, you asked about SCSI.
The chief reason for External SCSI arrays is simply that they can be expanded farther than the internal kind. Internal SCSI is limited by space in the server, whereas external SCSI is limited by space in the rack.
Back in 2000 I put together a 4-shelf SCSI array for an Oracle database. It involved 48 disks in 24 Raid-1 pairs. We needed both space and speed for this system, and the DBA liked this design. It was dedicated to this one DB server. Looking back, it was horribly over-speced for what it was doing, but hey, I was new back then and the DBA was a grizzled veteran.
A brand new system will use SAS for much the same thing, only it's easier to get multiple servers (known as initiators) to talk to the same storage (known as targets). SCSI was limited to no more than two initiators on a SCSI bus.