Do you make trees for wargaming or model railroad? Want the trunks and branches to have more realistic textures? This is for you. Mount it on a sturdy paper clip and roll it up and down while the putty is still soft. The bark roller will imprint a bark like pattern.
TheTerrainTutor have demonstrated this on his youtube channel (along with some others rolling pins):
https://www.youtube.com/watch?v=9U9-E7zfX3U
I encourage you to watch the whole video, its good.