Sometimes I'll serge a fabric that I know frays terribly such as satins and silks. If I'm using them in a quilt and/or wallhanging this helps from fraying as much while assembling. I used to wash my fabrics so would serge the ends before I threw them into the machine but I no longer wash before cutting, I prefer starching and pressing instead as that will also let me know if I have any bleeders and the pressing will shrink the fabric a bit too.