Read Kog'Maw's lore, I dare you. Basically, he appeared in Icathia, was told by a mysterious voice to eat things and go to Malzahar, did as he was told and joined the League. Small problem, since he's been in the League, he hasn't eaten anything at all on the Fields of Justice. So, in order to make in-game Koggles a little closer to his lore, why not give him a little, well, something?

When Kog'Maw is within melee range of a minion, instead of his normal basic attack animation, he eats the minion instead, swallowing it whole. The animation plays at the same speed as a normal attack animation.

It's nothing special, just a little idea to make the Mouth of the Abyss more mouthy and less projectile vomit-y.

There you go. Kog'Maw can actually eat things now.