For the average joe there it's just an option and personal taste, and it comes with its own tradeoffs and learning curve. Well integrated ZFS would probably be the main thing for average joe.

For developers, it is interesting to think of as a self contained toolkit. If you are building firmware, platform images for bare metal or cloud, it creates a much better demarcation than any attempts Linux can put forth. This is related to why you might like OPNSense. But if you are just a consumer it only indirectly matters to you.. consistency of build and product, quality of subset of network drivers and subsystems like pf to support your mission, ability go in and quickly and correctly fix the right problem at the right level etc.