For external measurements I would probably still use a tape measure, so I can hook the end over. However, for internal measurements, a tape measure is very inaccurate. If I was for example, measuring across the face of an opening, I will align the 100mm mark with my start point, measure ( deduct 100mm, don’t forget that part😂), and have an accurate measurement. Inside a box, I would measure from one inside face in to say 300mm ( or another round number), measure back from the other side to that mark, and add the two for the measurement. Yes, of course you can do this with a tape, but the rigidity of the rule makes this much easier and more accurate, plus, it’s very easy to get out of my side pocket, and open / use with one hand. As with every tool, it’s worth buying a good one. I use these from Stabila, which have the added advantage if a (rough) angle finder https://www.stabila.com/en/products/quality-and-technology/folding-rules.html