FSL isn’t the only supplemental programming you can do with 5/3/1.
There is SSL, BBS, BBB, and others that I can’t even remember. Supplemental work is applied to what your specific goals are. That’s what the strength of 5/3/1 is, strong core principles paired with the flexibility to apply to almost any situation.
The question isn’t what the benefit of FSL is, it’s what are you goals and what/if supplemental work is required or necessary for you to achieve those goals.
The new book is an excellent resource for this. It helps you to tailor your programming to suite your specific situation and needs.