In the #oggcasplanet IRC channel in freenode, Klaatu mentioned a use for this that more people might have.  I had it myself and didn't consider using this.

That use is to distinguish between the left ctrl and fn keys on laptops. For example ctrl is usually in the bottom left most position on HP laptops, whereas those two keys are reversed on Lenovos.

I've used this method to mark the ctrl on both. For what it's worth, I decided to use 3 dots of super glue in a horizontal line on the key because sometimes my finger hits that key in different places.

Chose to mark the ctrl instead of the fn key because the ctrl is the one i need to use most often and the ctrl key is not in the same position relative to the fn key on different keyboards.