There are different reasons you may want to create multiple Domains.
Organise Issues by topic
You can see Domains like a folders. You can organise your Issues by topic or by another category of your preference. The Issues will first arrive at the Domain connected to the Channel. From there, you can organise the Issues by putting them into different Domains. Refer to the picture below for reference.
Triage Issues to teams
You can also see Domains like teams. The Issues will first arrive at the Domain connected to the Channel. You can then triage Issues to the relevant team. Each team may be comprised of different people. You can customise who has access to the Domains. Refer to the picture below for reference.
Depending on your set up, you may lose or retain access to the Issue after moving that Issue to a different Domain. If you are part of the Domain the Issue has been moved to, you will retain access. Conversely, if you are not part of said Domain, you will lose access.