It also allows you to catch many errors up on the design board before sewing (and after), especially on large quilts.
The eyes can't take in a queen size quilt with multi-patched blocks in one take......they're darting all over.
When it's reduced to a much smaller image, it's easier to see if something is out of whack.
A camera or a peephole can also be used if you don't have or want to buy the specific tool.
I think it's one of the most useful notions in my sewing room - it prevents a LOT of potential grief.