On the plus side, the engine won’t be using any fuel so you will save money (but note that modern engines don’t burn fuel when you lift your foot off the accelerator while the vehicle is in motion).
However, there are a few disadvantages. The power steering won’t be in operation and so the steering will feel very heavy, particularly at slow speed. If you turn the key all the way off, the steering lock will come into play so you will have no steering at all. The brake hydraulics will also not function and you’ll need considerable force compared with what you’re used to in order to achieve any braking effect.
A manual shift car will slow down quite quickly if you leave it in gear with the clutch up due to the engine braking effect. A car with automatic transmission will travel almost as far as a manual car in neutral as the fluid flywheel greatly reduces the engine braking effect. Applying the handbrake will only make a marginal difference to the car speed. Depending on the model of the car, the lights, including the brake lights, may stop working.
Sooner or later the car will stop, either when it crashes into something because you have little control over the vehicle, or just through friction and gravity. All in all, it’s probably not a good idea.