The purpose of flaps is to increase lift and drag. At lesser deflections, they mainly increase lift. As you increase flap deflection, lift increases but so does drag. Beyond a certain point, any additional deflection just adds more drag.

You want more lift for one purpose, to be able to fly slower, and thereby land at a slower speed, which means a shorter landing roll. Very important for jet airliners and jet fighters.

You want more drag for two reasons: First, to increase your angle of descent. This enables you to fly a steeper approach while maintaining a given airspeed. Second, to require more thrust on approach. This reduces spool-up time in the even of a missed approach.

So why would you want less flap in windy or gusty conditions? Actually you really don't, unless you are using a very high drag flap setting. Airbus A320's and Boeing 757's use the same flap setting no matter what. What they do change; however, is approach speed in gusty wind conditions. The same applies to model jets. In gusty wind conditions what you need is a higher approach speed to account for a sudden drop-off in headwind component. The only time you would use less flap for landing is if your normal landing flap is a high drag setting, like 40 degrees. In that situation, you might want to land with 30 degrees because you don't want any more drag than necessary in a go-around, and you don't want to have to wait for your flaps to go from 40 to 30 degrees. If you need to go around in a gusty situation, a lot of drag is the last thing you need. You want lift.

So to summarize, unless you normally land with a very high drag flap setting, you don't need to change your landing flap setting. Just carry a little more airspeed on final in case of a sudden drop-off in headwind component.