Why put in front of the file name “_” or “_” in scss/css?

Question

Why put _ in front of the filename in scss?

_filename.scss - Why does it need _ ?

Answer 1

Files with _ (underscore) are ignored by compiler. However, all those files are imported into single, main SCSS file (i.e. styles.scss) which is actually the file that is compiled (it doesn't have _ (underscore) in it's name)

The final goal is to compile only one SCSS file, and to have only one CSS file as a result of that, which has various advantages.