Well, full time 4WD makes driving under certain conditions more stable. Wet or otherwise slippery roads, especially in turns are negotiated with more safety than in a 2WD. But you could of course slow down when in a 2WD - however, sometimes slippery spots sneak up on you and you don't have enough time to react, in those cases full time 4WD could be like life insurance.

1 The only 4WD concept that is truly useful on pavement is "full time 4WD".

2 AWD, or all wheel drive, is the same as full time 4WD - only difference is that AWD does not have low range.

3 "Part time 4WD", a system that is operated in 2WD most of the time and used part of the time in 4WD. Part time 4WD is pretty much only useful for off-road use. This setup, when used in 4WD on pavement, creates some safety risks: ABS will no longer work and it creates heavy understeer.

4 Automatic 4WD is a sophisticated 2WD system offered under misleading names like "True 4WD" or TOD (torque on demand). It should be called part time AWD. It works pretty good on pavement under slippery conditions but is absolutely useless off-road. A cheap solution with questionable benefits.

Not very many vehicles offer true "full time 4WD" - check with your manufacturer what they offer you.

Now, here is a simplified answer how 4WD utilizes traction better than 2 WD (and forgive my start at "Adam & Eve"):

Each of the 4 tires on a vehicle has traction (the friction between rubber and ground). Lets assume for arguments sake that each tire on dry surface has a traction value of 100 - a total of 400. **

In a 2WD only the traction of 2 tires (traction value 200) is used to create forward movement. Torque of the engine is using the resistance (traction) of two tires to get you moving. Lets say a torque value of 140 is needed to move the car. The traction value 200 is "used up" by 140 torque units. A safety margin of 60 remains.

Lets say due to slippery ground the traction value drops to 50 per tire. That gives you 100 usable units on the two powered tires - but with 140 torque units your "traction account "is overdrawn by 40. Result is spinning tires and you can't get the car moving.

In a 4WD system the engine's power is not sent to two tires (each carries 50% of the load) - power is sent to all 4 tires (each tire carries 25% of the load) utilizing the traction of the other 2 tires.

Going back to the previous examples, on dry surface with 400 traction and 140 torque withdrawal a safety margin of 260 remains (wow) and on slippery ground with 200 traction and 140 torque withdrawal a safety margin of 60 remains.Unlike the 2WD, tires are not spinning, forward movement is guaranteed. As you see, in 4WD the traction account is twice as high as in 2WD.

This long story only explains how 4WD uses traction better and helps to get a vehicle going when the ground is slippery. This is the situation when a 2WD can't take off on snow but a 4WD can. This does not constitute safety - it is only convenience. A 4WD can go where a 2WD can't.

** all traction and torque numbers were randomly selected for easier understanding of the subject.