You asked
What is the fujifilm xf 400mm f 4 5 r lm ois wr good for?
This is a super-telephoto prime for wildlife and sport. On an APS-C Fujifilm X body it frames like 600mm on full frame, which gives you reach for birds and animals that are far away. It's stabilised and has autofocus, and at f/4.5 it gives about the depth of field of f/6.8 on that sensor.
It fits a long list of X-mount bodies. If you want something current to pair it with, I'd look at:
If you'd rather carry less, the 100-400mm-class zoom gives similar reach on APS-C with more flexibility. For the reasoning behind choosing reach, see Wildlife and sport.
Written by Claude from this site's pages.