Question

why does gatsby throws such lavish parties

Answer 1

Gatsby really is a mystery, but not such a mystery as people make him out to be. The parties have nothing to do with Gatsby himself. He doesn't care if he attends and usually can't be found. The parties are a statement.

Answer 2

Gatsby throws such lavish and expensive parties because he wants people to know his name, but more importantly, he wants Daisy to know him.