[BigAls87Z28]

A trans with high milage, 100k or more, the tolerances have increased between the parts greatly. Dirt and metalic particles help this, and actualy help keep the tolerances tight like stock. Once you remove this dirty fluid, and flush it out via full fluid exchange, you now remove all these particles that might have been giving an artificial reading in line pressure, which will then cause the trans to slip and cause possible problems.
Trans Fluid Exchanges should be done as a maintence, not as a problem solver. If you have a slipping trans, you are going to need to see a trans shop. Higher milage trans service will only cause more problems.
Having it done ever 50k will increase the life of your trans, just as an oil change does.
Here is some food for thought. ATF and engine oil are pretty much the same thing, made along the same lines with just different treatments. Your engine oil is changed every 3k cause it breaks down and becomed sheared, and that oil is being tossed around an engine that can reach high temps. Trans fluid can sometimes experiance temps 2x greater then engine, especialy in trucks in towing situations, and is being pushed, squeezed, spun and shot around the trans, and people disreguard it as just to be left alone.