That's just a gimmick, as they do not sell the software, just access to it, ie you can use it as it is hosted on their cloud,
you have to bring your own delivery/SMTP servers, yet pay them a one time or monthly fee.
For some it may make sense, as they have no tech worries this way, but get less freedom/power compared to having your own install.
Technically, this will probably not work for long, as the infrastructure will most likely quickly fail.