Have you ever gone to listen to an orchestra? The musicians take the stage early and tune their instruments. Each individual instrument must be in tune, but the critical thing is for the entire orchestra to be in tune with each other. That's why they tune up together.
Now this is no surprise; I'm going to relate the orchestra to your vehicle. Your engine has many component systems that each need to be working properly. And the components need to be working in sync, all timed up together in order for the engine to work efficiently.
We call this... (ta-da) a tune-up. When an orchestra's out of tune you get a bad concert. When your vehicle's out of tune you get bad performance and bad fuel economy.
So check your owner's manual for recommended tune-up intervals.