Indeed. After I graduated from basic electricity, using light bulbs, switches, and batteries, my next step were several iterations of those X in 1 kits. The manuals for those generally explained how the particular circuit worked and gave a good basic introduction to electronics. Later on, I built some of the Science Fair P-Box kits, these came in a plastic box, one half of which was a red plastic perforated piece and the other a clear plastic (later smoked) cover, with the parts and instructions inside. You built it on top of the red part, inverted so the wiring was hidden underneath, and had the clear plastic top to fit over the components when not in use. I had a bunch of these - neon light sequencer, shortwave radio receiver, indoor/outdoor thermometer. The instructions supplied with these always explained the theory of operation as well.
In today's surface mount world it may be different, but it used to be that kits were less expensive than the built up model, plus there was also some satisfaction in "doing it yourself". Just like other hobbies, the kit concept seems to be disappearing. Too many people want instant gratification. It's probably the same in electronics, but in model building, the kit makers found that they could take their $5 kit, have it assembled by the same Chinese factory that was doing the injection molding for $1 per kit, and now sell the fully assembled version for $30. Netting a much higher profit PLUS no longer having to deal with people who lost, were missing, or broke a piece in the kit.