Well, one reason is if you use NFS. Whenever I have setup a device to use NFS (although the reason to do so is much reduced with the more recent versions of SAMBA), I have had to use the server's IP address to setup the mount point. If there is a way to do it without having that IP address, I am not aware of it...
I have the IP address. I just assign it in my router instead of in unRAID. unRAID used DHCP to get the assigned IP from the router. It is always the same, known IP because my router has that specific IP address reserved for the MAC address of my unRAID server. Same for all of my other devices. They all use DHCP but they all have known, unchanging IP addresses because I set them up in my router. Everything managed in one place but everything has a known, unchanging IP address.
True, you can do that. On my old cheap router (pre 2010), there is a table for up to eight reservations. (I have 15 addresses currently assigned on my router via DHCP and that does not include the two servers!) Of course, you have to know the MAC address. I know that the Network Settings page for unRAID servers has that information but then you have to explain to how to find both that address, the current IP address of the server that was assigned through DHCP and how to find the proper place in the router settings to be able to make the proper entries. Either way will work, you make your choice. Perhaps, you would like to write up an explanation of how to do the reservation method in any user's router that will be easy for the neophyte to follow?
I do it the same way as trurl, and I think that is the best way, since the router will not give a client that ip address over dhcp then.
In my old Asus router (Asus N-66U) and my Ubiquiti edgerouter, I can map the mac addresses to an static IP, but the router would tell me the hostname too if that is available, then I know which MAC address my unraid server has.