"Display grid lines at file view"

Why does the checkbox have three options: checked, unchecked and square?
And it seems to show gridlines only when the square is selected.

By the way, are the lines supposed to be this faint, or is it a problem with my windows settings?

Have you tried it? I have found these differences:
Unticked: plain background
Ticked: alternating background and lines in reverse colour to background
Square: plain background, but lines between columns and lines.

Exactly what's written in the help file:

Hmm, it doesn't show alternate colors for me.

Unticked: plain background
Ticked: plain background
Square: plain background + grid lines

Check Windows colour theme?

No, it's not that.
I thought that it was the fault of Windows 8 UI because Microsoft made everything low-contrast compared to Windows 7, so I made some screenshots on my laptop with Win 7 to show you the comparison. But when I got them on the PC the grid lines were just as invisible, so it seems it's a problem with my display's gamma settings.

EDIT: Sorry, forum's spoiler tag doesn't work with images.