This style modifies the width of the Gmail navigation box (the area that contains "Inbox," "Starred," etc.). The default value seems unnecessarily wide, so the (non-auto) values set in this style reduce the width to the minimum value possible in order to retain visibility of all the elements and maintain a balanced appearance. Of course, these values can be changed to whatever you'd like.

This isn't necessary for wider monitors, but is of relevance for smaller monitors. Gmail (old version) - navigation box (float) compliments this style well.

...I do have to admit that reducing the width of the navigation box does potentially reduce the visibility of contents of the labels box.