I like to design different outfits and hairstyles quite often for my characters, so I have to do a reference sheet for any substantial change, or else I'll get confused/miss details. It also helps prevent me from drawing characters in too similar of clothing, haha. I try really hard to have each character have a very specific style they wear, maybe that's just me.
They're also interesting to see, since they're all drawn more or less in the same pose, it's fun to see how the character has changed over time when you compare the different reference sheets together.