I use the 4WD lock button as insurance. The electronic 4WD is always on, it distribute torque to each wheel thru sensors. Normally the RAV4 is in front wheel drive and delivers torque to the back wheels when you are accelarating or when sensors detects slip. If you manually lock the 4WD system, it will bypass the sensors and lock in the wheels at 60% front and %40 back if I remember right.
I use to have a MDX with the same system, the one on the new RAV seems to work better. I went thru a few unplow street with snow drifts up to 2 feet. I didnt want to get stuck since I didnt know the RAV4 ability yet. From what I understand, the 4WD lock button can get you out of trouble even after you get stuck, but I didnt want to chance it and I wanted to get familar with the 4WD lock function as well.