I use the flat flower pins whenever I need to hold fabric together securely while cutting with my rotary cutter and ruler. You can put your ruler right down on them and they won't make the ruler go all "wonky" while you're cutting. They really came in handy when I was making my Stack and Whack quilt and I had to pin in many places at once and cut with the pins in place.
I also use them to mark rows. Especially when the rows all look very different from one another and it would be easy to get confused when it came time to sew the rows together. I mark the flower with a number using a permanent pen and place the pin either on my design wall next to the row, or on the row itself up in the left-and corner to keep the rows in their proper order.