To have fun and relax. Some are educational (brain age & my word coach), some give you some excercise (Kinetic & Wii Fit) and others are all about puzzle solving or flat out action.

soley for entertainment. your interaction with them triggers enzymes in the brain to cause pleasure. This makes videogaming enjoyable.

They can be many things.
Sources of entertainment, method for relaxation, something to help you think, etc.
Depending on the type of games you usually play.
I play games because I like how they tell stories and if you want to see the whole story you are given the challenge of playing the game right to progress the story further.
Or in MMORPGs I like how they make me think and concentrate on tasks.
I also see games as an art form so playing them just shows my appreciation for that art.
I can also learn life lessons and tasks from games whether they are fictional or not.
Some people tell me I have no life because I like to play games so much but I feel games are much more than just sitting in front of your TV with a controller in hand.