I added these because I wanted to denote which of these roads are urban and which of these are rural, something I've been doing in Germany for quite a while.
In fact, I even see this as even more important because locality and built-up area signs are detached from each other in Poland so you can't even tell the status with these (not to mention all the instances where "highway=city_limit" is placed at its exact location and not at its legal functionality).

Mind you, I didn't actually notice that no values have actually been defined for this key BUT(!) (and this is what made me rather perplexed about your comments), this has nothing to do with speed limits (since speed limits through signs can appear both in- and outside settlements and there's no way to tell where a road ultimately belongs), only to mark roads inside built-up areas, especially when there are other rules besides speed limit which differ if the road is urban or rural.
I'm open to another solution to mark streets inside "obszarach zabudowanych" but please, don't just claim "zone:traffic" is a speed limit marker because. It. Simply. Isn't.