mainly for focus reasons..... at F22 you should get a nice clear shot....(because of the dof).... but at a low F number you run the risk of the IR lightwaves putting your focus out.
Some prime lenses have a dot on the focus ring especially for focusing with IR (so you can focus the shot, turn off auto focus, attach the filter, and then adjust the focus ring to compensate for the IR lightwaves.)...... This should ensure your shot is in focus too when you press the shutter..... but i just prefer to shoot at a high F number just in case.
It also does give you a longer exposure time.... which can be fun and add interest to the shot. The only time i would reduce the F number is either when its really cloudy and dull...... or if your attempting an IR portrait, or something which requires a faster shutter speed.