Go by the manual to start. It gives you the basic purpose of each gear, and when to use it.
One main advantage of slipping down to D4, and even D3 is to save some wear on the brakes when descending a steep grade, and you want your speed reduced for safer handling, or when towing a load.
Of course the lowest gears are intended for SLOW driving, especially when pulling a heavy load, descending or ascending steeper than typical grades, poor road surfaces, etc.
Myself, I always keep my automatics out of the overdrive range in stop and go traffic, where the transmission tends to shift needlessly in and out of overdrive. To save some wear on the tranny. The higher engine revs tend to offer more cooling also.
You might do the same on a winding road where there are a lot of switchbacks, and you can never attain high enough speed to use the higher gear(s).
For just running down the avenue and jumping on the freeway, I just keep it in the highest gear.
I think the principles are the same for most all vehicles, the top two gears in the MDX are higher (overdrive) than most though.
Of course I am not covering all conditions like, uh.. well, racing (ill advised!!) where depending on the vehicle it is better to shift manually though the gears, or let the automatic do it's thing. The automatic in the MDX was not designed to be a 'sport shifter'.
How it gets used, is another story....