grammar - Reason for "the" as whole class

I am learning uses of articles.

And the book i am using tells that

"article the is used when a singular noun is meant to represent a whole class"

For example
The cow is a useful animal.

I want to know the reason how the above example represents a class.
How can I prove it represents a whole class

This question seems foolish but I am really confused please help me

And correct me where I am wrong