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

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.