commercially prewound bobbins hold googobs more thread than you'll ever get onto a bobbin using your sewing machine.
also, they are extremely convenient.
naturally, they're also a bit pricey. :lol:
i only buy prewound on reusable bobbins. if they don't have that little hole, i pass them right on by. i have a big box of prewounds for embroidery. i stocked up on cotton prewound in neutrals for piecing. i use the empties for colored thread so i can coordinate top and bottom when quilting.
i get mine here:
http://www.terradonembroidery.com/clear_glide_15.htm
at just over 50 cents per prewound bobbin, they don't cost much more than brand name empty bobbins and thread separately. the time they save is the HUGE value added.
the class 15s are reusable and will work in almost all machines that use a class L. the reverse is also true.