Explain the extra gears in an automatic to me

Went up the hill (Denver speak for "the mountains") and my keep starts spewing smoke. Pull over in a little town (Idaho springs) and the mechanic tells me that my model jeep has a pressure release off of the tranny (ha-ha) that when it gets hot spews tranny (ha-ha) fluid. He then told me to use 3rd gear. I have used it more often and it helps.

I used to consider myself a gearhead but never gave fucks about automatics so I have no idea why 1 or 2 is needed when I can just put it in drive. Can someone elaborate on the intricacies of automatic transmission gears? Phone Post 3.0

Low gear high gear. Up Hill down hill. Pulling, etc. Phone Post 3.0