Why Replica disk load?

Question

MongoDB

I inherited a Mongo 3.6 cluster with running out of disk space. It was useful to see what was done there and a lot of questions .. for some reason in each shard there are 4 replicas (3 normal and 1 hidden) + arbitr. There are no backups, why is hidden at all - xs. But the main question is that for some reason the disks are very heavily loaded, on read and write, and including hidden from which it should not be read at all. And on one secondary replica Disk IO usage is generally 60%, while on others including primary 30%. ssd nvme drives can't be faster..