Why, in a nutshell?
All the old 1.3, 1.2 apps will play happy? What is the win? Thanks.
bp
Faster file and directory operations, rock solid stability, slim to no chance of any filesystem corruption, small memory footprint, still actively maintained, open source, very optimized code and Toni Wilen and Piru are both working on it which is a +75bazillion in my book.
It rocks basically!

edit: Your apps will be blissfully unaware of what filesystem they are running on btw. apart from Disksalv type tools I guess, but there is PFS Doctor for that. not that you'll need it very often if at all.
You'll find the tools and docs in this archive but don't use the filesystem handler from this one as it won't work, use the PFS3AIO handler I posted earlier.
http://aminet.net/disk/misc/PFS3_53.lha