I wonder if it includes workarounds for Google IMAP bugs etc. Some years ago I migrated over 10 years worth of emails from Google to Fastmail and I ran into a lot of issues with Google's buggy IMAP implementation. Things like a random 1 in every 1000 email failing to delete so I would have random emails still remaning. I found out it was a known issue which Google pretty much decided they WONTFIX. I later found Fastmail had its own migration tool that (I presume) has workarounds for various bugs in other providers IMAP implentation making it easier than using e.g. Thunderbird to do it. If I could go back in time and use a tool like this that has a way to work around Google's bugs that would have saved a lot of my time.

I moved from gmail to fastmail a few years ago

and being the cynical sort of person that I am, I didn't trust the fastmail importer

so I ran it, and also wrote my own implementation using the gmail api (NOT imap), and another using the fastmail jmap api, and reconciled them

100% match (bar the "Muted" folder, which fastmail ignored)

pretty much perfect