It seems pointless to me. My source is the Pokemondb page, so did Pokemaster make a mistake?
Imagine this: You've recently hatched a Spiritomb. - By default, it would have Pursuit / Shadow Sneak (default) / Spite / Curse - If any more than two egg moves were passed down, then Pursuit and Shadow Sneak would not have been learned. The moveset would be like this: Egg move 1 / Egg move 2 / Spite / Curse - But if alongside the two egg moves, you had also passed down Shadow Sneak, then the moveset would be like this: Egg move 1 / Egg move 2 / Shadow Sneak (egg move) / Curse
See the difference? Those were the two ways by which Shadow Sneak could be gotten on Spiritomb (other than Move Reminder, because he is not concerned here). commented Aug 30, 2015 by Qwerty_Zoom
You could have just answered. commented Aug 30, 2015 by sumwun
If a Pokemon learns a move by levelling up, it also becomes an egg move. Take Spiritomb for example. It learns Shadow Sneak by level up. So, if you put it in the daycare with Shadow Sneak, the offspring will also have Shadow Sneak.
Hope this helps!