i think Dunster nailed it-
when you want to-
when you've finished that big wonderful quilt & you don't want to deal with (man-handling) it through your domestic machine-
when time is an issue-
when you don't like quilting- you like creating
many people love making tops-but have no interest in quilting them- they do not want to just stack the tops up, they want to get them done- so sending them out is a good option.