The Porkchop Express was used as the main transportation to get from Piggy Island to Bird Island where the pigs are looking for exotic foods: The Eggs!
When the Pigs went off the ship, Red, Chuck and Bomb decided to sneak into the ship to see what is really going on, and it has a Dress-Up room, a storage room for all the piggy vehicles and a trampoline room. Bomb jumps on one of the trampolines so hard, that it broke, and revealed a bunch of pigs underneath the trampoline room.
As time has passed, more pigs came to the island, crushing Red's house even more, and even vacating the beach and inside his house without permission.
As the pigs were leaving the island, they boarded the ship and took the eggs and Billy the Sign in one of the ships, until Bomb and Red tried to break the chain, but it was unsuccessful and the two hit the water before they could escape.
When the birds were entering Piggy Island, they discovered the harbor with a lot of either restored, or broken Porkchop Expresses before stopping at the sand to reach to the King's Castle.
The last level of the Bird Island episode has a structure that strongly resembles the piggy ship from the movie, it even came with a boulder anchor that can be dropped on top of a house that's just right under it!