The object of the game is to use up all your letters and all the letters in the middle of the table and the person who does this wins, which only means you can stop playing and admire the interesting, funny and even misspelled words that everyone came up with. Then you'll want to play again, and again, and again... The feel of the letters in your hands and the sound of them clicking on the tabletop add another element to this addictive pastime.
Variations are listed on the tiny instruction sheet that is included in the pouch and if you're all alone but feeling wordy, you can play Banana Solitaire.
More info here: bananagrams-intl.com and some history in this video: