The whole game are made out of "screens". When scrolling cross border of one of those screens then game tries to spawn enemies on that screen. Enemies spawn only in particular ram slots. If that slot occupied by enemy or pick up then enemy wouldn't spawn even if all other ram slots are available. Also when enemy spawn in particular slot it tries to move in the same direction as previous enemy in the same slot.

For example i have troubles with that crawling guy near energy tank. If he moves to the left he blocks the way when i try to go back after E-tank and that could cost me a lot of time. So just by watching at map i know that he spawns in slot 2. And previous enemy that was spawn in that slot was that jumping guy on the ceiling. So by killing jumpin guy while he moves to right i can make so crawling guy near the energy thank will also move to the right.

So if you have troubles like that in the runs you can try use that map to manipulate enemies. Just find in what slot problematic enemy spawns and what was previous enemy in that slot. So you can try to kill previous enemy in certain way to make other enemy move to different direction. Or you can try to keep previous enemy alive, so when scrolling cross the screen with problematic enemy it wouldn't spawn. By using this map you can know exactly whats going on and there is no need to guess what to do or something like that

edit: also those small fly-like enemies works differently. You can't manipulate or despawn them

via https://www.speedrun.com/metroid_nes/thread/m9412/1#wbq1w