Take a conical plastic bag, slip a nozzle in, nip off the bottom to expose the opening in the tip &
then you pipe with the bag, However, say you started piping with a shell tip and decide you want to use a star tip, you can't swap tips with the same bag without digging through all that icing in the bag. Messy messy. You would need a bag for EACH tip. And if you want to use two or three different colors, you would need three bags with 3 of the same tip.
Enter "couplers" stage right. Slip the base part of the coupler into the bag first, place the star tip over the coupler base, screw the coupler ring on to hold the tip in place, fill with icing and start piping.
Now, to swap tips to a shell tip, just unscrew the ring holding it onto the base, pull off first tip (star), add second tip (shell), put ring back on and tighten it. Continue piping! No mess, no fuss.
To swap to a different color icing if you only have one star tip, remove it from one bag, rinse to remove icing color and then use with second bag of colored icing that has the coupler on it.