I use them as a learning aid...helps drive home what size feathers I should be looking for when I'm trying to tie a specific size. When I try (even using a hackle gauge) to select a feather from a normal cape/neck/etc, I always seem to be a little off.

The 100's packs are much closer. If I want to tweak the size up/down, I grab the next size pack up or down and use it.

Sometimes I intentionally go up or down a size and it's handy to have the 100's packs to crib off of.