It gives you a fairly small gain in fuel mileage, but comes with several downsides.

For one, it’s harder on your suspension system and gives you a rougher ride. This will wear out your struts faster, which is kind of expensive.

Next, it lowers the footprint of the tire with the ground. This means that you have decreased road grip and it will take longer to stop in an emergency.

Lastly, it means your tires won’t last as long. The tread will wear uneven and it may cause tires that should last you 50,000 miles to only last 35,000, for instance. A rather large waste of money due to how expensive tires are, now.