Batman relies on science, tech and his wealth to fight crime as he has no super powers. As a result, and to make the character more lifelike, “all of the tools have a credible basis in scientific reality” – Christopher Nolan.
Christopher Nolan is a director who is known for building a lot of the elements in his movies, and not relying on computer graphics and effects. So for Batman Begins, Nolan and designer Nathan Crowley built a working vehicle. So the Tumbler you say in the movie was a working machine.
The design inspiration came from a cross between a Lamborghini and a Humvee.
“Something that would have the presence of almost a tank like vehicle, but also with the speed and grace of a sports car” – Christopher Nolan
Design elements of the Tumbler are functional and based on scientific reality – like the flaps. Based on airplane flaps which gives a plane lift, the flaps on the Tumbler pushes it down allowing it to go faster. There is also no axel between the front wheels. Each front wheel is totally independent – one can stop, while the other is still rotating – allowing the Tumbler to spin around.
This video gives a look at the build and design of the Tumbler: