I'm not clear whether cold and excercise are capable of increasing the number or proportion of brown fat cells, or whether they just trigger activity in existing brown fat.
The impression I got from the article was that cold triggered distinct brown fat deposits in particular places (neck, back, etc.) to burn fat (and sugar?) as a way to increase temperature. By contrast, the article implied that exercise turned “white” fat into “brown” fat, spread throughout the body rather than in particular isolated spots. It wasn’t clear what the purpose of this exercise-related brown fat was.
I'm just guessing as wildly as Doc Brown's hair, but it seems logical that the "white" fat is the storage form of fat, while the brown one is the kind that body uses to do actual useful stuff. Like in a gasoline engine -- the gas is stored in concentrated form, but it's diffused and mixed with air moments before the actual combustion, as that's more efficient.
So body creates the exercise-fat to have a ready source of energy required for physical activity (remember, while humans were evolving there were no exercise, only physical activity needed to survive).
Cold apparently activates the brown fat - causing it to burn fat and sugar. Exercise seems to release a hormone that causes white fat to convert to brown fat, but they don't seem to know why exercise causes this yet.