A fun thing they show in the animations is how you can do basic arithmetic in the size fields. So if you've setup your document in millimetres (I do this for woodworking designs) and you have a shape with width 5mm you can do this:

    5.000 + 2mm

Or:

    5.000 + 0.2cm

I don't know when Inkscape started doing this, but I just gave it a go one day because it felt like something I would put in (and to their credit, they did).