I think the most common use is to secure the edge before adding binding.
It's possible to piece quilts with a serger, but the seams are bulkier (and not all patterns can be done that way).
I have seen a serger used also for decorative stitching on a quilt top. One denim quilt was serged together with stretchy red thread on the top, then the red seams were sewn down flat on the sewing machine. Very cute!