On most airplanes, you can only extend the flaps to a certain point without hearing the horrible gear warning horn. So, on the 767 for instance, you can extend flaps to 20, and fly along with the gear up in silence. But once you select flaps 25 or 30, the obnoxious gear horn starts and it cannot be silenced without lowering the gear.
Also, the gear is noisy so you don't want to extend it untill you get withing 5 miles or so. Thus, you'll work the flaps out to about 20 degrees, and then wait till you need the gear, then drop the gear followed by the rest of the flaps.
That keeps it nice and quiet.