Procedural Foliage Spawner and Landscape Grass were developed at the same time, for the same project by epic.
That was in 2015 for the Kite Demo, their open world push.
Why were they developed and released together?
Because they complement each other .
Landscape Grass is great for smaller things, since it is all processed on the GPU. Downside is no complex processing and no collision support. Upside is it’s incredibly fast to spawn a lot of instances (think a grass field or lots of small little rocks in a mountain area)
Procedural Foliage Spawner works on the CPU and allows for more complex simulation. It happens on the CPU, so it supports collision.
The downside is it’s much slower and needs more memory to store it.
Both LGTs and PFS/FTs have a role to play. Hence they were developed together and complement each other.
If you are not trying to make huge, detailed outdoor open worlds that are densely populated - then you might not need them working in tandem.
But Epic Games themselves saw a need to create both of the systems and these systems continue to be used by many of the industry leaders 