Eating a balanced diet is very important 这里的“eat”为什么要加ing?

题目

Eating a balanced diet is very important 这里的“eat”为什么要加ing?
Eating a balanced diet is very important.
Eat a balanced diet to stay healthy.
这两个句子看起来差不多,为什么一个“eat”要“ing”?一个不加呢?

答案