It's required according to our historical SQL structure design (based on PostfixAdmin) and Postfix settings, otherwise normal mail user will be considered as non-exist account and mails sent to users will be delivered to catch-all account.
In Postfix, we have below settings:
virtual_alias_maps =
proxy:mysql:/etc/postfix/mysql/virtual_alias_maps.cf,
proxy:mysql:/etc/postfix/mysql/domain_alias_maps.cf,
proxy:mysql:/etc/postfix/mysql/catchall_maps.cf,
proxy:mysql:/etc/postfix/mysql/domain_alias_catchall_maps.cf
virtual_alias_maps is triggered before virtual_mailbox_maps, so if mail users aren't caught in virtual_alias_maps, it will be considered as non-existing account. With this SQL record, all mail users and aliases will be caught in first SQL query map (virtual_alias_maps.cf).