Because you don't have to build a feature in your own software that

- stores the templates

- selectively runs campaigns and tracks open rates etc

- create campaigns, see how many users are in there, have a safeguard before you hit "send"

- actually a nice interface.

Let's assume the "open my database to the internet" is not a dealbreaker, then this tool gets your development cost of 1-8 weeks (depends on how big the code base is) down to a 19 USD / m. Depending on where you hire / how much your time is, your opportunity cost of several thousand dollars for the feature are now much smaller.

I think there's a niche but strong case for this.

[edit: typo]