Scraping Caramel for Flan
(note : I'd have posted this as a comment because I don't have personal experience w/ this, but people keep complaining when I do as this might have enough information to be considered an answer)
I don't know about this case, but in other times when you're cooking down sugar there's always a fear of 'seed crystals' getting into the sugar (causing it to become grainy).
A little searching seems to support that theory.  There were many websites talking about seed crystals, but I'm going to quote the one from Fine Cooking (The Science of Caramel), as they specifically mention problems with the sides of the pan:
http://www.finecooking.com/item/60729/the-science-of-caramel :

What can go wrong when making caramel?
The caramel turns grainy
The biggest drawback to the wet method is that the sugar tends to recrystallize more easily than it does with the dry method. When the sugar and water boil, sugar syrup may splash onto the wall of the pot, where it evaporates quickly and forms back into sugar crystals. If even one of these crystals falls back into the syrup, it can seed a chain reaction, turning the clear syrup opaque and grainy. Should this happen, you can remove it from the heat, add a few tablespoons of water, return it to the heat, and stir until the crystals dissolve before continuing on. That said, it's better to keep recrystallization from occurring in the first place, so here are a few ways to prevent it: ...

See the article at Fine Cooking for advice on how to prevent seed crystals and other information on caramel.