@eibhear This was discussed on our marketing channels, but the main reason is to make users more aware of how new/old their version is. Something like "LibreOffice 7.6" doesn't really say anything – but "year.month" has more relevance to end-users. We had requests for this, and some other open source projects use it too (most notably Ubuntu).
Of course, one version scheme won't satisfy everyone, but we think this is more meaningful for end-users.