Why do sass files that are inserted into the main file via import start with an underscore?

Question

Sass

I read in the documentation that this is necessary so that the compiler understands that this is a piece of code and does not create a separate css file.
https://sass-scss.ru/documentation/pravila_i_direk...
But the bottom line is that in my project, despite the names, with or without the lower one, only one file is connected to the head, I don’t see other files.
Please explain what is the difference?

Answer 1

for example, create 2 files: styles.scss and main.scss
And run your compiler.
If compiled correctly, you will get 2 files at the output: styles.css and main.css
But if one of these files had an underscore at the beginning, the css file would not have been created