In a nutshell, Device Offset essentially saves you having to use blank columns.
For example, let’s say you have a Visual Composer Column that is 6 columns (1/2) wide and you want to centre those 6 columns on the page. You would then use an offset of 3 columns (1/4) to keep it centered. Essentially emulating 3 blank columns in front of the 6 you’re trying to position. Make sense?
Visual Composer is a powerful plugin but sadly the documentation is awful.