On the 94 Jeep Grand Cherokee what's the point of having part-time and full-time 4wd?

Full time means you can run 4x4 on all surfaces. Part time means slippery surfaces. Full time applies power to the front wheels. Part time applies power to the front wheels and LOCKS the front wheels and rear wheels together. This means the rotation rate of the front and rear wheels are the same. If you turn left or right you change the rotation rate of the front wheels and this means something has to slip, or break. If you are on slippery surface then the tires will slip, if you are on a dry surface the tires will try to slip, chirp usually, and hop. This puts strain on the transfer case and cause damage to the gears or the drive chain. Full time 4x4 is great in the rain. Part time 4x4 is great for off road, mud, lose soil, etc...

On any vehicle not only JEEP that have multiple settings is based on what the manufacturer has determine to give you the best grip for that vehicle at specific conditions. All the settings are is a percentage of power that is distributed to the wheels that are not the main driving wheels. In a jeep I believe is the front wheels that are the main driving wheels and the rear are the secondary driving wheels. What that means is that when you are in 2wd your vehicle only uses the front wheels and when you use any of the other settings with 4wd you are simply adding a percentage of the power to the rear wheels for power at what the manufacturer has tested the vehicle will perform the best. You are also right to assume that damage will occur to your vehicle if you left it on a particular setting, it might not be the case but is always better to be safe than sorry. Since there are multiple ways to have the set up of AWD some are restricted by gearing and if yours is that means that if set on the wrong setting and forgetting will damage your AWD system. You do have to remember that your vehicle is a 94 and AWD systems have come a long way since your type. By the workings of the AWD such vehicle will always use up more fuel than a non AWD identical vehicle, however as technology gets better that difference is almost negligible, we are talking about a few litters per tank(half a gallon) compare to about 8 to 16 litters (2 to 4 gallons) for your vehicle. Hope this helps some what.

It sounds like you already have a perfect understanding of when you should use which setting. The "extra" part-time four is just a bonus for when you might be spinning a wheel, or when you are off-roading, so that you can reserve the 4-lo for when you are actually stuck. More options is better, and was worth paying extra for, as it gives you better ratios, and locks the center, so that front and rear axles "act as one" and so you can get better use of the engine's torque.

part time locks the centre diff and is for use if you get stuck somewhere and just need to forwards or backwards as it will kill the transfer case if try to turn corners,Full Time is for everyday use and can be selected on the move and will drive the front wheels as well as the back but will turn as normal! hope this answers yuor question

so one can use it when one needs it you can switch full tiem four wheel drives use more fuel drives slower high range for higher speeds but still might need all four to pull four low when you need to crawl and a slow pace and dont really have to switch if road slick drive slower leave it in four wheel drive high drive slower

part time is meant for when the road is mixed but mostly dry at least on my 01. Also you really don't need 4 wheel drive in the rain if have good tires and don't drive like a jerk.

The part time setting is for serious driving in adverse conditions. Mostly it is intended for off road use, but if the snow drifts are two feet high and rising, use it. Just keep in mind it is best to switch back to full time when you come to clear highway. Unless the snow is really deep, the full time setting is perfectly adequate. My Cherokee Sport broke through drifts up to the headlights in just full time so I really don't think you need to worry. If you should happen to end up off the road in deep snow or mud, the part time locks all four wheels together so you get maximum traction. That is the only time I would use mine.