Porgs main reason for being added to the many star wars creatures is, so the island has wildlife, but they were also added for the entertainment of the people watching, almost everyone fell in love with these little creatures as soon as they saw them. (love at first sight.)
I think they may have also been added to help chewie with the loss of Han solo, who died, the porgs that chewie brought on the the falcon are his new companions on the ship. The main porg, the one that is thrown into the window during The Last Jedi (This scene was so sad and hilarious.) His name is Terbus. In an interview it was confirmed. (If I get the chance I will add screen shots.)
I also think that porgs may have been added to cause a problem, I have read many articles that say porgs could bring chaos because it is unnatural for them to not be on their island, they will put off the balance of the solar system. Cause more/ other animals might try to eat them (nooooo) or if they eat other creatures.
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Porgs
HumorThis is almost completely opinion based, but there are some real facts that are in here, (facts found in The Last jedi Dictionary.) No hate, Porgs don't like hate. Enjoy :3