Well exactly as you say: it’s a single service instead of having to combine multiple. In my case dovecot was a lot faster for my mailboxes, but postfix was a piece of shit and I was happy to get rid of it and the many components (rspamd, dkimproxy, etc.) it required. It has far too many footguns, and I shot myself multiple times with them over the years. So the most important part (SMTP) is significantly simpler and IMO better with stalwart. And the mailbox part hopefully evolves as well (it already has JMAP, so that is already an advantage over dovecot as well).