Why does the transmission fluid have to be changed in a 2006 Jeep Liberty when the owners manual says the transmission fluid installed at the factory is good for the life of the car unless contaminated with water. Normal wear and tear on the transmission will make it dark... so why would the manufaturer write that in the owners manual?

If fluid is dark,then yea ignore it,by changing fluid now,could save you from having to replace it later on.The owners, manual was wrote by some one that cant, see the future,by changing bad fluid make,s tranny run better for you and get a longer life span.

I think a lot of people will agree, that heavy or busy use isn't going to make a difference unless you have heavy mileage like I do. 200000+, when I had the filter and fluid changed. I don't know what others think but this is what I know and have been told by more than just mechanics

In the Haynes manual for my truck (97 C1500 Chevy Silverado) it says that if the fluid at the end of the transmission dipstick is a dark reddish-brown color, or if it smells burned, it should be changed (replaced, presumably along with the filter); and if you are in doubt about the condition of the fluid, you should compare the trans fluid to new trans fluid, for color and smell.

Other commentary: Re: your owners manual says, under normal conditions it should never need changing FOR THE LIFE OF THE VEHICLE, unless contaminated with water. I'd say that the life of the vehicle, as meant by the owners' manual writers is likely to be the point at which it becomes more expensive and time-consuming to repair than what you're willing to put up with. Or it might be when the first visible rust shows up and the value of the vehicle drops. Anyway, might not (to them and a lot of others) a dead transmission signal the end of the vehicle's life? And perhaps ignoring the transmission fluid's condition will result in transmission "death" at about 5 years, whereas changing it "when needed" will likely extend that to, say, 8 or 10 years. In either case, when the transmission "dies," it may mean to them that the vehicle "dies" with it, in which case one could probably say that it's a given that under normal conditions it should never need changing FOR THE LIFE OF THE VEHICLE unless contaminated with water. And the contaminated-with-water provision is only needed in case [in court] such contamination might be considered "under normal conditions" in flood-prone areas. Re: particles from the gears causes it to get dark ... I'm just a backyard mechanic and have limited detailed knowledge of transmissions, but I think (and may have read) that the more likely source of the darkness is the disks that perform a function like that of the clutch disks in a manual transmission. I'd guess that metal shavings wouldn't remain suspended in transmission fluid too long; but I c/b wrong. But I'd also think that they'd not turn brown or dark in a transmission. The disk material, OTOH, probably is dark or would become dark in the process of being worn off of the disks. Just sayin'.