Integration with Email Mailers

When using a third-party service, you will not be limited, for example, by gmail restrictions (restriction on sending a certain number of emails per day / hour, etc.) - the mailer has more flexible settings (depending on which one you send through). If you have your own domain mail that allows you to send a lot of letters and not get banned - you can use it.
For mass mailings, they mainly use intermediaries - this allows you to work with large volumes + settings for external services (mailing management, checking how many letters were opened, how many reached, how many were sent to the wrong addresses, etc., etc. depends on the service).