Some knitting patterns may ask you to work a "provisional cast on". A provisional cast on leaves that first row of stitches "live", allowing you to go back to that first row of stitches at a later time and pick them up without the bulk of the cast on edge....ie...without having a seam and/or making for a neater "join" between two pieces.
There are a number of ways to work a provisional (temporary) cast on, but this is the one I find easiest and quickest.
Sometimes I use a provisional cast on for the body of a sweater if I'm not sure what type of edge/ribbing I want to commit to....for instance on a fair isle sweater once the body is knit and I can actually see how the colors look, I might want to work the edging/ribbing in different colors to bring out/highlight a particular color to balance the design. Or on a cable/aran knit pattern that called for ribbing as the edge, once the sweater is done, it might be that I think I'd have preferred a seed stitch edging rather than a ribbed edge ....so if I work a provisional cast on and knit the body first, I can wait to choose the edging until after the sweater is knit, then I can choose the stitch I think looks best/suits my needs best...even if it isn't what the designer chose to use.
Another little advantage of working the edge of a sweater after the body...especially on the cuffs....is that if the cuffs get worn or dirty, it is really easy to pull the cuffs out and reknit them if you've knit them last
I didn't include this alternative approach to working a crochet chain provisional cast on into the video below, but I'll verbalize it here. You can also work the crochet chain AROUND THE NEEDLE to begin with. So holding your knitting needle (under an armpit, on your lap, between your knees...whatever seems comfy), place the slip stitch on the knitting needle to begin and then first chaining on one side of the needle and then the other, crochet a chain stitch directly onto the needle. This technically saves the step of picking up the stitches after you've crocheted the chain. I find it equally fast and easy to just work the chain and then pick up the stitches as it is to work the chain around the needle. But for those of you interested in trying the alternative method, just work the crochet chain around the knitting needle to begin with!