Routing domains, or VRFs, are probably easiest to think of like VLANs, but one layer up.

The two typical scenarios are;

1. You want to use the same IP space multiple times across different networks (i.e. multi-tenant)

2. You have a bunch of different networks that you really never want to ever be able to talk to each other (you can allow it through routes, etc. but by default this does not happen). Although you can achieve this with VLANs and ACLs, on a single routing table, rules can become a real pain in the ass to manage very quickly

You could use this for stuff like guest or IoT networks.