We are very familiar with vehicles that have traction on all tires, lets agree that its 4 wheels. Most folks want traction on 4 wheels in our climatic conditions.
Ever wonder how it came about?
The classic vehicle with traction on all the wheels was usually a pick up, or a Suburban, or the early versions of what we know today as SUV's. In most instances the vehicle had a 4 speed truck type manual transmission with first being the creeper gear, a 2 speed "lockable" transfer case that could accomodate a PTO (power take off), ideally a positraction in the rear, free wheeling hubs for the front wheels, and for extreme applications a positraction units would be installed in the front differential.
The rule of thumb was narrow, tall tires to have ground clearance (do you remember 7.50x16 truck tires), accompanied by skid plates, especially for the gas tank (s) and the areas with the least ground clearance.
If you are concluding that these vehicles were horrific on the highway you would be correct! If you concluded that they would endure a good amount of off road abuse, with regular weekly maintenance to tighten most components that were rattled loose during the week...you would be correct again.
How did these vehicles evolve to what we understand today as an AWD vehicle?
 The 4 speed manual with the creeper gear began evolving towards a 3 speed automatic, easier to off road with an automatic, less tiring to plow snow. Imagine how fast this vehicle would accelerate with the transfer case in Low, the automatic shifts all its gears, put the transmission in Neutral, shift the transfer case to High, the transmission in D, keep on going.
 Free wheeling hubs (disconnect the front wheels its was literally a 2 wheel drive vehicle) were next to go, totally unpleasant to forget engaging the front wheels and having to deal with snow, mud, rocks, one wheel refusing to engage.
 The driver at the time had an innate understanding of the various sounds a 4x4 would make in its off road progress, and usually if it started "shuddering" in its forward progression especially in snow...backing out was a wise decision.
 For years it was body on frame, solid axles, transfer case configuration, although the leasure vehicles were more luxurious, and 4 wheel traction and a higher stance was a major priority compared to off road capabilities.
 Yes...the classic 4x4 still exists mostly in pick ups.
 Manufacturers have converted the "street 4x4" to an AWD configuration responding to the need for luxury, traction on all the wheels, excellent highway behavior, independent suspension front and rear, and technology hading the task of distributing power to the wheels with traction.
 The transfer case...still need "something" to compensate for the differing speeds of the front and rear wheels, took a while for the transfer case on street version to evolve from 2 speed to single speed, and then the transfer case becoming a "clutch pack" to allow slippage and compensate for the speed difference.
 The street version is an AWD vehicle that delivers power to all 4 wheels, with computing power and AI controlling how the power is delivered to the individual wheels. In most instances its a unibody structure (no longer body on frame), with a FWD configuration (transverse mounted engine).
 The technology deployed to deal with the myriad of variables of a current AWD vehicle be it SUV, CUV, or car is truly impressive...from precisely controlling the torque going to each wheel, to balancing the torque from the rear to front several times a second to name a few.