[WelshPaul]

what does the "Synchronize to Kodi" function do?

Don't know, I've never used it. I guess it does the same as 'Library Update' or 'Scan for New Content' does within Kodi, which is the way I go.

Using it for albums is very time-consuming if you have a large collection

I agree with this if you're creating all your nfo's from scratch. I've only ever used it to edit and correct things after I've exported the nfo's initially from Kodi.

it is not clear to me whether there is any value if you have perfect tags?

For me (being a little compulsive about things) the combination of tags, nfos and .srt files gives me the greatest ability to flexibly store many different bits of information, and then I can design my own visualisation screens to present it as I want. This is really useful for playing along to songs, etc. I know there's been a recent major overhaul and increase in the availability of infotags, but it used to be the case that you couldn't easily present song tag infos on screen but could from the nfos. I guess it depended where the database scanned the information from. So, it is probably becoming less and less important generally, but I still like the flexibility to tailor things the way I like them, and that's what I've always loved about Kodi.