Saves you about ~2GB of RAM with no catch, that's it

Just like what it said this mod basically reduces your memory usage...

How does is do it?

Well, for starters, in Minecraft, every block has a state, and certain block checks the state of it's neighbors too. This interaction allows the creation of fluid and it's interaction (like lava and water => cobblegen), and other other big things that i genuinely still try to comprehend. And these things, they take RAM.

This mod basically optimizes things around that and certain other issues. Either by replacing certain methods with one that measurably takes less ram (in this case FastMap), implements caching for certain things, performs deduplication, and some other things.

In total and in most observations. This mod basically saves you around 2 GB of RAM. This is literally an essential mod (say that again?) especially for big modpacks, just put it in your pack and forget, all benefits, no catch, compatible with everything.