Why?

Because when the filter is clogged with lint, the amount if air that the blower can deliver is greatly reduced.

When airflow over the heating element is reduced, the heating element runs hotter. Overheating a dryer element dramatically reduces the life span of the element. Cleaning the lint filter regularly just makes economic sense to save excessive wear and tear on the heating element.

Low airflow also means there is less air circulating to dry the clothes. That means it will take longer to dry your clothes. So you will spend more on electricity.

And excessive lint buildup is a fire hazard!

So, be safe and smart. Clean the lint filter in your clothes dryer before every use.