WHen I get the white inserts out of the protectors the first thing I do is put an "X" on one side so I don't accidently use them in a layout and pick one up instead of my white cardstock.
Then I use them to start all my layouts. I have over 30 layouts on one of my tables right now that I am working on and each is on a piece of the white insert paper.
Once I do that, the colors jump out at me and I start getting ideas. Once I start arranging them on the white a bit I get ideas for how I want the layout to look and can get the paper I'm actually going to use. But when I lay them out on the insert paper I get a good idea of size of photos and how manhy on a page and what I want on it. That does more for me that laying the photos I want to use on the table.