I don't get Email Routing

I don't get Cloudflare Email Routing.
It can only be added to domains without MX records.
What are those domains? Why would anyone have domains without MX records, other than "parked" or empty domains?
Normally you use Cloudflare with the primary, company domain, where MX records are definitely configured already, with Google Workspace or similar.
How would you use Email Routing with those?
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