this is really a neat feature, I've been in the forum for a long time and this is the first time I am knowing about this, thanks for sharing.
@OP looks like your question has been asnwered by tranthidung sharing the "search for member" feature". you can also search members by their rank not just whether they are a staff in the forum or not.
This is a feature for searching users with usernames, that is most common use case, but there are two other helpful ways:
Searching for forum admins, global moderators, staffs as pointed out above.
Searching for VIP members, donator members.
For general searching by ranks, I don't see it is too helpful because search results are too wide.