Earlier quoted context omitted.
You can set up a wildcard/catchall alias instead of having to manually create an actual alias for each service. Someone can send email to anything@yourdomain.com and it will go to your inbox. You can also send with a wildcard - the big desktop email clients, as well as the FastMail web interface, as well as FairEmail on Android all let you type something for the part before the @, so you can send from anything@yourdo…
You can even do more than that. I have different categories per services: paid services go to @paid.mydomain.tld, social logins go to @social.mydomain.tld etc. So that means something like paypal@paid.mydomain.tld or twitter@social.mydomain.tld. Edit: using Route53 for DNS subdomain wildcard.
Any specific reason to do this over the usual way (paypal@mydomain.tld, twitter@mydomain.tld)? How are you benefitting from the extra hierarchy?