So I use these all the time in garment making, but I've been using them MORE for leatherwork. While a plastic edged ruler is always going to come off worse against any of the blades we use, I find these excellent for lining up, and squaring up pattern pieces, finished goods etc.
Although my steel ruler has 0.5mm marking, even a small variation can create a line that isn't perfectly parallel (or square) to and edge/element of the pattern. They work best with the printed side down.
Just a note that not all brands are created equal. I've seen reviews in the sewing community where some of the printed lines are either not perfectly square and/or are slighly off in their measurement. These are the only ones I've used so I can't recommend a particular brand, but I think these are fine (?)
I also use a ruler like this, absolutely love it! Although for some reason I can’t seem to find a metric one. 🤷♂️