Well, for the most part, drum maps aren't absolutely necessary.
As Cactus noted, he has never used one.
I mostly use them because it makes editing in the PRV easier.
Also, you can split out the drum notes to different synths. This isn't possible without a map.
For instance, I like the behavior of the Session Drummer 2 Hi Hat better than the EZDrummer Hi Hat.
So using the same input device (usually my Alesis DM6) and MIDI track, I can trigger the SD2 Hi Hat and the rest of the kit can be mapped to EZD.
It's just a routing tool, really. You don't have to use it, but it's good to know how it works.
You can trigger SD3 with your DR-880 or A-Pro without a map just fine.
But suppose you have a few snare hits that are a little behind. So you want to adjust them forward.
Open PRV, and you see....piano keys. Hmm..which one is the snare? If you use a map, it is clearly labeled.
By the way, it has nothing to do with the multi outs. That is the audio from the synth. The map, if you choose to use one, only affects the incoming MIDI data that triggers the synth.
Just keep doing what you're doing. Push buttons, try stuff, see what happens, and ask questions if something is confusing. At some point, it all starts to click. I knew jack about MIDI when I started. Getting a good grasp of how the signal flows through the machine really helped me to understand how it all works.
It's second nature now. Easy, breezy. You'll get it.