These biamp cabinets in their day were the end result of a trend in the 70's where IF you wanted more horsepower than the SVT, the big Sunns, or the Acoustics, there were no amps. These were also the days when folded horn 18's roamed the Earth, most bass cabs had 15's, and 2 or 4-10's hadn't been thought of.
So guys began to run bi-amp: Get a preamp (back then, IVP's and the like), set it atop a commercial power amp (DC300's LOTS of CS800's, and others) and some pair of cabinets (sometimes PA bins, assorted bass guitar bins) and have at it. Some of these big rigs sounded great, and a lot of them were fairly iffy, and ALL of them were heavy and hard to move. Remember, built-in passive crossovers were only good for home hi-fi back then, bass cabinet design was not taken very seriously for players, and only a few companies like JBL built serious high power speakers.
Peavey had the idea to build these 1820's and 3620's and sell them with an amp (somebody help me remember this amp) that was essentially one of their bass preamps united with a CS800 in one chassis. This was a bitch to move, but it really moved some air, was smaller than separate components, and in the right hands sounded just fine.
Fast forward to today, with lightweight, stressed construction cabinets, neo speakers, and 1000 watt amps that fit in a cigar box, and how styles and preferences change over time as well as the technology, the market has just moved away from this being a choice for most guys. The choices for bass amps now vs. when I started in the 70's is as big a jump as a DC3 to a Boeing Dreamliner. If you'd have told me then that a Genzler rig with 2 of those 12" Bass Arrays would eat my old Acoustic for lunch, I'd have never believed it.