I always thought they were a waste of time... and they were the way I had learned to use them. I was always taught to just use a scrap of fabric and put it under the pressure foot when you get to the end of a seam you are sewing. Then snip off your piece (or seam) you have just sewn. Sew another seam and snip off your scrap and put it under the pressure foot and start the process again. That little scrap of fabric was used over and over again until it was so thick with thread the needle wouldn't go through it.
Then I took a class from Bonnie Hunter of Quiltville a few years ago and my life changed forever. She has 2 whole books with quilts in them that use nothing but leaders and enders pieces. What you do is pick out a unit to make such as a four patch unit, half square triangle, nine patch... etc. Then when you get to the end of the seam for the project you're working on, use one of those as your leader (or scrap). You're making something useful, not wasting fabric, not wasting thread, saving time but not having to go back and clip threads, and eventually you have enough units to make another quilt.
My unit on my leader and ender project is nine patches.
It happened quite by accident. The last two quilts I've made I calculated it incorrectly and I cut out twice as many nine patch units as I actually needed. I've always wanted to make a postage stamp quilt so here we go.
I put the nine patch units together and pinned them. I put them next to my sewing machine and grab one for the leaders and enders.
I have a whole pile of nine patch leaders and enders now.
I will need 30 nine patch units per row and I will need 40 rows so that's 1200 nine patch blocks. I will put them in stacks of 10 and pin them together until I get to 30.
Then I will put the 30 in a stack and pin them together. I don't want to go ahead and put them together in rows because I want to be able to move them around and play with the layout.
It takes just a little bit of planning ahead of time but it's well worth the effort in the end.