The different widths are used in the code so that different content can be shown/hidden at different widths. They are basically triggers.
Resize your browser Window. You'll notice the appearance of the site change as the width shrinks. The widths that trigger these changes of appearance are basically the wide/medium/narrow widths you see in the style properties.
There is no need to change these values. Based on what you want, those default widths are more than adequate.
All i´ve done now is what your screen suggested. But i have done so many Style Changes that are somewhere fixed width settings.
So i will setup 1.2 new, delete all Addons before and import my Database and reinstall the Addons. Maybe a bit complicated but atleast safe