What are frying pan ridges for? I expect frying pans with raised ridges are for a few purposes such as adding "grill lines" to meats and keeping food out of grease somewhat as it collects.

Why or when should I use a frying pan with raised ridges?
Whenever you want grill marks. Or, perhaps, if you want to ensure fat rendered out of meat drips away. Why you would want to do this I do not know.