Why do I need to replace the spark plugs in my Audi A4?

I recently purchased a used Audi A4 and I'm not sure when the spark plugs were last replaced. Can you explain why they need to be replaced and how often it should be done?

Spark plugs are an important component of your engine's ignition system. Over time, they can become worn and dirty, leading to reduced fuel efficiency and performance. It's recommended to replace them every 30,000 miles to ensure your engine is running at its best.