This is an exact scenario that just happened earlier. Also as a note, the rng for what iv's are passed down are set for every egg before hand, and rejecting an egg resets the rng.
Say I want to get a 31/31/31/xx/31/31 Jolly Breloom and I start out with a 31/xx/31/31/31/31 Male Adamant Shroomish. There's three things I need: A Shroomish that is a female that has 31 iv's in attack and a nature of Jolly. So I breed the starting Shroomish with a Jolly Ditto (with no perfect iv's) and get a Jolly Shroomish with 31 iv's in Sp. Def and Speed.
Now that I have a Jolly Shroomish, and with a couple good iv's, I pair it up with a Ditto that is 31/31/31/31/xx/xx. So I put them in the day care, get 5 eggs, hatch them, and take them to the iv judge. I notice that 4 of my 5 eggs all have the same stat: XX/XX/31/31/XX/31. That means it's taking two iv stats (maybe three) from Ditto as seen by the two 31 iv's, and a 31 iv from the Shroomish. Notice how it does not have 31 iv's in attack. That means that the attack stat is not getting passed down by the Ditto, so I need to reset the rng for an iv distribution that passes down the Ditto's attack stat. Rinse and repeat a couple of times and then boom, I get a female jolly Shroomish with xx/31/xx/xx/31/31 which is exactly what I needed to get my perfect Breloom.
So yea, that's why I use that method.
