300f2.8 is probably the most popular of the “Big” lenses on other systems, 200f2.0 replicates the same FOV and effective light gathering with APS-C (accounting for the inherently worse high ISO performance on crop sensors).For wildlife one of the examples I seem to hear about is shooting exotic birds in the jungle, you don’t need excessive reach but due to the foliage blocking out most of the sunlight the lighting conditions are dim.
It’s ideal for indoor sports or where the arena is smaller (e.g. Basketball or Tennis), or on a larger field if you only have one position to cover.
And then music and stage events that normally allow reasonably close proximity to the subject.
People like to shoot portrats with 200f2.0 on Full Frame but Fuji would need to make a 135mmf1.4 for that. This is probably not going to be used for portraits very often (not that it can’t be, you just get a lot of compression, good for someone with a long nose).
400f2.8 is the most common “Large Field Sports” lens, Fuji would need to make a 300f2.0 to cover that position (yes, it’s a 150mm front element, that’s the largest element Canon and Nikon are willing to produce).