For context I'm sleep deprived so sorry if I've missed something obvious, but what differentiates this with something like Fastmail's masked email offering? I use unique emails for every service I use and it works beautifully. This seems... more complicated? I under there's a price difference, but little else.

It aims to be simpler. It's not a mask. It has forwarding as an option but the idea is that you can use one of the inbox types for different needs. The one I use the most is prefix, because you create it once and then you can tack on any suffix to it. When the first sender reaches you, that suffix become locked to their domain (or account, if it's a gmail, outlook, yahoo, etc). That seems pretty useful for subscriptions. For one-time codes, just create a temporary one every so often. It has the option to delete itself (and optionally take the email with it).

Masked emails are infinitely shareable eg. a spammer can harvest that email address and then reach you. You don't typically have a setting to block anyone else (other than the original intended counter-party) from reaching you -- unless you set up some specific filtering rule (if that's even supported).

I recently found out there is a limit to the number of masked emails you can have on Fastmail (1000, iirc -- definitely not infinite for practical purposes). Also, I miss the option of being able to create masked emails on all the domains I connected to Fastmail -- you can only use the "main" one.