The transmission is what directs the power of your cars engine to the wheels at a steady speed And, to provide the best balance of performance and fuel efficiency, even under the most extreme load or Impact Ratingtemperature conditions, up to 2,000 moving parts of that transmission need to be lubricated, and cooled constantly.

The transmission fluid should be replaced every 15 to 30,000 miles.

Running with old or not enough transmission fluid causes the transmission to work harder transferring engine power less effectively. This can cause fuel efficiency drops of over 10%. Without sufficient lubrication, this will lead to very costly repairs or even complete replacement.
Repair Impact
Having your transmission fluid serviced regularly will extend the life of your transmission and prevent a transmission rebuild or complete replacement which could cost $2500 or more.

From a Safety standpoint, if your transmission is unable to shift reliably or shifts erratically, you might not be able to go faster than 20 mph and it could cause an accident.

And although it may not have a high environmental impact, leaking fluids are harmful to children, pets and other animals.