why would I tag a domain?
You might want to associate a domain with a customer/project if you have a lot of them. Or you might use the API to automatically do things to linodes, domains, or other services based on the tags it may or may not contain.
I sent some feedback to Linode on this. It would be great if domains could be grouped by tag. The 'group by tag' view apparently exists on lots of other list pages but not domains… which seems odd, since even if you have only a couple of Linodes you might have lots of domains for various purposes.