Also, sergers use a LOT more thread than a conventional sewing machine, so we buy those big spools simply for economy. You could use regular spools of thread (I have in a pinch), but they'd run out PDQ.
Serger Thread vs Sewing Machine Thread
I get my thread from www.Jaycotts.co.uk now.
I have been buying the economy Moon overlocker thread in small 1000 yard spools from them to get some nice colours. I had already bought the big 3000 yard cones in White, Black and Cream for tidying seams from local shops.
I did a couple of overlocker courses and on one of them used Wooly Nylon for rolled hems. This is a brand name for texturised nylon and is also known as Bulk Nylon or Flock Thread. It fluffs up and fills the gaps between the stitches to give a more solid effect. Anyway I bought one in black and one in white but I confess I have never used it!
Hope this was useful, though it's probably a bit late as your message was last November!
Karen's info is excellent...................I've used woolly thread when serging napkins and tablecloths....It works like a dream as you don't need to do any other sewing/hemming...........works for me.............