I combine the a7cII with various small primes (Sony 24mm f2.8, Sony 40mm f2.5, Sigma 90mm f2.8, Voigtlander 35mm f1.4) and I have a very capable camera that fits in a tiny bag. It's great for street and travel.
Then when I need to I can attach the same body to a big lens (Tamron 28-200, Sigma 100-400) and slap on a Smallrig grip to counteract the lens weight. It's great for protests and birding.
IMO the benefit is that one can purchase a single body and do big camera and small camera things with it.
The a7cII has a built-in EVF and IBIS, though, and it’s priced similarly to the FP2. Autofocus on the a7cII is probably better, too. I can definitely see the appeal of the a7cII even though in the end I decided to go with a camera system with smaller, less expensive lenses.
Whereas the FP2 seems to compromise more on useful features in the name of compactness.