"Empty use" of can and could [duplicate]

Question

When I got confused with the meaning of the phrase "empty use" I searched on the internet for the meaning. I found a section in the book CBSE(Central Board of Secondary Education) entitled INTERACT IN ENGLISH WORK BOOK-Class 9. In this book under Unit-4 (Modals) they say, "Empty use of can and could." Below this headline they gave some examples:

1. I can smell a fire.
2. I can hear a weird noise.
3. She could feel her hair standing up.

Now my question here is why they used the phrase "empty use." I need to know how to use modal verbs in depth because my first language is Kannada so it's very difficult to learn this aspect of English.