Its a VERY useful tip in my opinion. :D
It would probably be used most in selected the aircraft and getting rid of those extra edges around the aircraft or whatever when selecting and copy/pasting.
Select your whole object after you've cut out the background, then click on Selection, Modify, and Contract. Pick a reasonable amount of pixels, I usually go with 1-2 and contract, then select inverse, and delete! viola, nicely edged aircraft