@font-face bolditalic file why?

Question

css

I want to connect the Open Sans font to the site, I downloaded it from fontsquirrel.com. There were many variations of fonts in the kit (light, extra, semi).
I included only 3 files regular, bold, italic.
Please explain why, for example, connect OpenSansBoldItalic if such a font can be obtained by applying:

font-weight: bold;
font-style: italic;

The same situation with other files.