I don't get it. Why is it so important that the mockup is also cut on the bias when making hose? Every tutorial I've found so far put great emphasis on doing this but does not state why it should be done.

Cutting straight vs on the bias changes the stretch characteristics of the fabric. For hose (which need to fit closely) they should be cut on the bias. The purpose of a mockup is to give you a detailed pattern of exactly what it needs to be for your legs, so you should cut it in the same way (on the bias) so that it has the same stretch characteristics etc. Otherwise you can get the fit perfect on the mockup, make your actual product and find it doesn't fit you properly.

oki, thanks!