Agree in part, but remember that formal verification need not be done in full. By analogy, we don't avoid testing simply because everything under the sun can't be tested. Even simple things like verifying that certain API endpoints are idempotent, or as a few steps up, that the datastores used by Facebook have distributed consistency and fault-tolerance properties, are of enormous utility.
Exactly, it feels dishonest that this point is so rarely brought up in essays on formal methods. You can do things like prove that all possible faults are always caught, or any memory that is accessed has first been malloc’d, or that the API endpoint will always respond (liveness). These are often both easy to specify and difficult to guarantee with conventional testing.
1) With formal verification you can prove properties about your system. With extensive testing you can also do the same but you need to take care and make sure you cover all possible cases.
2) With formal verification you can easily test things that are normally hard to cover by traditional testing. For example, if you want to test if that a postgres randomly goes offline, you don't lose any data, it's pretty hard to cover with extensive testing, whereas with formal verification you just say "postgres exists, it has state and can be online/offline, it can go online/offline at anytime" and you're covered.
To a first approximation, formal verification can guarantee some property holds for every possible run of the program, rather than just the tested ones. It's a lot more powerful than it sounds, because this unlocks the ability to talk about qualities of programs that cannot be tested (effectively or at all). Hyperproperties like confidentiality, integrity, and availability tend to be quite difficult to test, for instance.
