It might be for spec ops guys deep behind enemy lines and maybe short on ammo, and thus may need the ability to scavenge and use Kalashnikov rounds from neutralized enemies. I doubt that separate barrels would be issued to all and sundry, and thus it would only be a factor for a small cadre of users that might need such capability.
I agree with your comments. Necessity is the mother of invention. I would add that the sound or noise signature of an AK47 when fired is quite noticeable and distinct. By having the capability to convert to an AK 47 round, perhaps the sound from this FN weapon will be similar to an AK47 and which will will permit our spec ops folks to not be as detectable in a firefight. Every little bit of surprise and stealth helps.