Another important reason to convert on-premises mailboxes to mail-enabled users is to retain proxy addresses from the %BRAND_EXCHANGE_ONLINE_SHORT% ...

Another important reason to convert on-premises mailboxes to mail-enabled users is to retain proxy addresses from the %BRAND_EXCHANGE_ONLINE_SHORT% Online mailboxes by copying proxy addresses to the mail-enabled users. This lets you manage cloud-based users from your on-premises organization by using %BRAND_AAD_CONNECT_SHORT%. Also, if you decide to decommission your on-premises %BRAND_EXCHANGE_ONLINE_SHORT% organization after all mailboxes are migrated to %BRAND_EXCHANGE_ONLINE%, the proxy addresses you've copied to the mail-enabled users will remain in your on-premises Active Directory.