If you drive in 4-L or 4-H on dry pavement, your driveline will get wound up: the front and rear wheels cannot turn at different rates going around corners, etc. and tension develops in the shafts and transfer case. If you do it enough you'll break something eventually. Not to mention that the thing will understeer like crazy.
On slippery surfaces the wheels can slide and relieve that tension.
Wet pavement isn't slippery enough to use 4WD on an ongoing basis, in my view. If I were on a wet boat ramp, or in a similar situation, then it's appropriate. But not for general driving.