I once had somebody come up to me at a gig and say that he was sorry I had to play so much brushes that night. Now, the gig was with a vocalist, but I don't ever recall her telling anything about how to play, let alone what implements I should use. Nope, I would say a good 90% of the time, I'm plying brushes because I want to. I may want a quieter groove or I may just want a different colour for that tune/intro/soloist. I'm not sure if I've made this abundantly clear but there are way more sounds available to us with brushes, and barring buzz strokes, we can also play all the stuff we play with sticks as well. Some of the things we can do are....
Articulated taps ( "whipping" the drum with the brushes )
Legato taps ( playing toward the drum horizontally as well as vertically. Jeff Hamilton is a master of this. )
Other legato shapes ( lines, triangles, z-shapes, all usually done while staying on the drum. )
Rolling the brushes across the drum to make the brush almost act like a rolling pin.
Accents created with the handle of the brush while still keeping the wires on the drum
Creating "buzz-like' sounds by hitting the rim of the drum with the handle, then bringing down the wires as they rebound.
Brush flams ( running over one brush with the other )
Choking up on the brushes a la Vernell Fournier to create less sweep sound but a more stick like sound and response.
Depend on the type of brushes, using the handle to create rimshots ( in the case of wood or extremely hard rubber), mallet type effects ( with softer handles ), or triangle imitations ( with the metal handle).
I'm not actually going to post any examples of these types of playing, but I would encourage you to seek them out. ( Hint: some of them are even posted on this blog ). Have fun!