It's my understanding, and it could be different now, that STCs were plans and machines that could independently manufacture things, without the need of dedicated AdMech presence. Things you would need on a new world could be built, without needing to teach the natives, or fresh colonists, how to do that, and such. Thus they were standard, and the Ford-made assembly line would crank out numerous copies of the same lasgun, Leman Russ, or air-scrubber. Some of them are the only place left where info on how to make what they do still exists, and the Imperium's ability to promptly mass-produce ANYTHING is ALWAYS in question, so they are always valuable. Again, that is my take on them, and subject to error. Venkelos said this.
That's pretty much how I figure it. Apart from that what makes them valuable is not the Mass Producing but the lost technology inside. Even if the designs are for a coffee maker. The tech of before the Horus Heresy even before the Great Crusade. During the Golden age of humanity. Priceless. Another difference is that I don't think that STCs can 'build' things as such. More just a keeper of designs. If they could build... then why that crummy reward in DW.