Internal combustion engine cars don’t consume gas at a constant rate. You can think of it like as you press the gas pedal you’re making the fireman on a steam engine shovel more coal into the fire. The harder you press on the pedal the faster the fireman is grabbing shovelfuls of coal and tossing it into the fire, and the harder that engine has to work. The more the car can just coast with just a tiny bit of fuel tossed in to maintain speed, the less fuel goes into the fire.

A relaxed driving style where you slowly accelerate, brake gently, and don’t speed/change lanes a lot means a lot less time pressing hard on the gas pedal and a lot less time burning lots of fuel. Also gentler braking will mean less wear on your brakes and tires so they may last a bit longer (tires and brakes are wear items that need to be replaced regularly, but the frequency is impacted by your driving style)

Such a relaxed driving style is also much safer because it gives you and other drivers more time to react and it’s more predictable to all road users