School me on torque converter lock up and lock up switch

Been reading for awhile off and on but I'm having troubles understanding the use of the lock up switch I've read so many times use to lock up the torque converter. I've read about people wanting to be able to to go up hill and lock up so that they can keep a lower trans temp. I don't know what it's like with deeper gears and larger tires but with my 3.55 axles and 31"s, if I place mine in to 3rd before a hill I know it locks up by the way the rpm drop a bit. I've read that ours also don't lock up till 40mph which I can attest to as in either 3rd or drive it does lock up once I hit 40, are guys wanting to lock up sooner than 40? If so why?
I know lock up does help with fuel efficiency, lower temp, and engine braking(little confused but know enough to understand)
Just trying to figure why the switch is useful and such

The whole spinning thing inside of the other spinning thing and fluids fly around and some how makes power to tranny confuses me lol