They were actually very
useful in their time, which was when wire-wrap was in common use, for both prototypes and production equipment. I've also worked with equipment that used them as programming jumpers, phase-locked loop filters and timing components. (In sizes from 8 pin to 40 pin DIPs)
The ones with covers were often used in production equipment. Where I worked, after we got the correct values soldered in and tested, we'd fill the cover with potting compound to seal them and attach them to the header. That kept the customer from tampering with them and altering calibration. But I know that some companies just glued the covers on.
The best headers were made of fiberglass or phenolic, they handled the heat of soldering better and were much more durable. The plastic ones were a bit of a pain, easily overheated and damaged. The trick was to keep the pins cool and held rigidly in place while soldering. A socket was the easiest way; I used to have an un-etched square of double-sided circuit board with several sockets soldered to it for heat dissipation. Another method was to find a thick strip of metal 0.3 inches wide, place it between the pins and clamp the whole thing in a small vise to dissipate the heat.
I haven't used any of them in probably 10 years, haven't even seen any for sale anywhere.
I once had a job to maintain about a hundred devices which each contained between one and four 4x4" wirewrap boards just crammed full of 7400 logic and headers. The people who built them weren't very conscientious about assembling them and bad wraps, cold solder joints and occasional wiring errors made them a challenge to troubleshoot and repair.
I don't miss that part of my career.
Edit: These seem pretty cheap and they look like the good ones I bought years ago.http://eolsurplus.com/Components.html
search for component carrier.