This is a very handy tool when you're painting armies... I don't know about you, but remembering how I painted something 6 months ago (sometimes even a week) can be tricky. So get a notebook and keep simple notes about how you paint something if you expect to have to paint it again at some point. (Or even if you don't)
I usually don't bother with this when I'm painting single models - BUT, armies look really nice when they're painted with a uniform colour scheme. Some variances are okay, but generally, you'll want them to be similar. Those similarities are what you should keep track of.
For example, I have notes for every version of the Eldar Alaitoc armour I've painted, and their accessories. (For reference, there've been 3 - heavy armour w/ chainmail, a newer style similar to aspect armour, and the newest sleeker style.)