I'm surprised Apple supports custom email domains to any extent. Apple is a consumer electronics company. Hosted email on custom domains is very distinctly not a typical consumer behavior.
Me, I've experimented with migrating some of my domains from self-hosted to iCloud+, simply for the ease of management and reducing the headache from SPF, DKIM, DMARC, etc. It turns out I have a spurious DNS record that was causing all my headaches, which I had forgotten to remove from older experiments, and I didn't need to migrate to solve this.
In either case, it's a nice convenience feature. Despite the trend, and the HN-think, Apple DOES do things for power-users sometimes.
Offering custom domains for personal can link up to office documents and this can make its way into business use.
For small businesses already standardizing on Mac hardware, it’s natural they should want to get the privacy and in-ecosystem utility of an apple version of Gsuite.