Hi
A serger will stitch, trim and finish the seam allowance all in one motion. It's great for fashion sewing and for working with fabrics that fray a lot, and can lend a decorative edging to your seams or sometimes for a hem or sleeve hem. I've seen some "ruffled" hems on tshirts etc made by purposely stretching the fabric as it goes through the serger, that sometimes look great...and sometimes not.
For quilting I would very rarely use it as it does add some bulk to the seams and if you ever have to undo/redo it's touchy since you have a lot more stitches to remove from that tiny seam allowance.
They can be very tough to thread, depending on the machine (I cheat and tie my thread ends together, pull them through until they get to the needle, then untie and thread the needle....saves a lot of work).
So if you do a lot of sewing other than quilting--- fashion, drapery etc it might be worthwhile. Otherwise probably not.