Why is legolas in The Hobbit movie?

Thorin and Gandalf had a discussion which eventually led to the expedition to Erebor. Thorin was to be the next king of Erebor and therefore was able to round up all the dwarves in the area to go on the adventure. He turned out to be a poor leader and it was a good thing Bilbo went along. Thorin was good at giving speeches and was a great warrior, even though he died because of his wounds in battle. Thorin's desire to retake Erebor set the events in motion that led to Smaug's demise.