How do i use the "Hi/Lo" "shifter" in this 1991 Toyota Land Cruiser, what is it for? Why would i change it? I have driven this car for a long time now and never use i do i need to?

The hi/lo is a function of the 4 wheel drive on the vehicle. The hi ratio would be used when negotiating sand or mud for instance and the lo for steep inclines where more power is needed. This is like having a gear with better pulling ability than first gear. You will find driving in 2nd gear lo will give you the same speed as 1st gear high.
Do not engage the four wheel drive system when driving on hard surfaces ie tarmac or concrete. Use it only on surfaces where additional traction may be needed. It is a good idea to use the 4 wheel drive system at least once a month in order to lubricate those parts of the 4 wheel drive system that would otherwise be "dormant" when in 2 wheel drive.
If you have diff lock funtion on the vehicle it would be a good to idea to engege this as well but once again only on a soft surface. The reason being that the centre diff lock tends to bind and stay engaged due to it not being properly lubricated.
If it does bind(dash indicator stays lit) find a patch of grass or sand, engage reverse and drive a couple feet. If this does not disengage the lock turn the steering fully left or right and drive in a circle. This should then disengage the difflock.