I don't want to unload my quilting machine from the frame if I want to sew something.
And different machines have different features - my big brother is a straight-stitch, super fast machine, so I piece with it because I only need straight seams and I want to go fast. My little brother is slower and has zigzag, buttonhole, and other decorative stitches, so I use it for those stitches. You really don't want to go too fast when you're stitching buttonhole stitches.