if you have some compelling reasons, I would appreciate seeing them, if you don't mind sharing
I don't think I could go as far as to say 'compelling', but I've found a number of little improvements and niceties that added up to enough to make it worth upgrading for me (that, and not wanting to see the developers 'go under', I suppose, given how much I rely on Path Finder day-to-day).
The new modules system is very flexible, but also I've found it to be more stable and reliable than the old way of choosing what you wanted to appear in each of the available 'slots' in the main window. For example, in my usual layout, I have the bottom 40% of the window taken up with a Preview pane (two-thirds) and Info pane (one-third). In Path Finder 7, to get these the way I wanted, I had to fiddle with settings for 'hidden views' and frequently re-adjust the spacing, and whenever I wanted to use Dual Browser mode, things would break (e.g. the wrong pane's item would appear in the Preview, or different items in Preview vs Info). But having defined them as modules in Path Finder 8, they have been much more reliable, always staying in the 'correct' positions and sizes, and never showing unexpected extra views or failing to track the selection appropriately.
There are also various visual things that seem improved -- contrast between text and background, font choices/sizes, divider lines and so on all seem a bit more 'right' in PF 8 vs PF 7. I hadn't really noticed these while using version 7 (or I'd become used to them), but on switching it seemed to fit in and be somewhat more consistent than previously.
I believe version 8 also has Dark Mode support for 10.14, but I'm a 'light mode only' person so I haven't seen that myself.
There are several other little things, not all of which spring to mind at the moment (e.g. I find the display of Shared in the sidebar is more useful than it was, in that you see more items and can therefore do more things without needing to go to the separate 'Connect To…' screen; and more fonts/colours/etc. seem to be customisable than in previous versions throughout the application).
Somebody else mentioned the 'Compress and Email…' command, which I don't use (since I use MailMate rather than Apple Mail), but I just tried it on a couple of sample inputs (one file and one application bundle) and it seemed to do the right thing for me (i.e. created a Zip file in a temporary folder, then opened Apple Mail with a new message open and the file attached). I'm not sure whether this means whatever previous problem there was with this feature has been fixed, or that it only works in certain circumstances…
As for the Favourites vs the Finder's list, I actually didn't realise that they were synchronised in version 7, since I had set up a custom sidebar configuration in order to get coloured icons anyway. The early comments about this seemed pretty clear: Apple provides no official API to do this, and the Path Finder developers didn't want to keep using unsupported or undocumented APIs that were unreliable and likely to disappear at any time, so they decided to remove the feature. So I wouldn't anticipate it coming back (at least not in its previous form; the most I could imagine would be something where Path Finder has an option to display the Finder favourites, but they're read-only in Path Finder). As I mentioned, this isn't an issue for me personally, mainly because I use Path Finder in 'exclusive' mode so I'm never interacting with the Finder's favourites list; the only time I'd see them would be in the sidebar of open/save panels, and I rarely use them there because I do most of my open/save interactions with Default Folder X (most commonly the 'click the Path Finder window I have open in the background to switch to it in this panel' feature).
Not sure if these comments will be of any use to anyone, but there you are! 
. The paragraph above addresses — but does not answer — his question, "How have your workflows changed?" The only answer I have to "Why upgrade to 8?" is: eventually you'll have to. In this case, the old advice "Never install version "WholeNumber.0" applied strongly.