At what stage of development should we use Formal Verification?

I am newcomer!

My question is simple. What is the sole purpose of formal verification? Normally, we do software testing after implementation phase. At what development phase should we employ formal verification techniques? for example model checking. Should we first formally verify our designs (system designs in finite state machines) and then start coding and later testing? or After implementation we do both verification and testing? I am some confused here. Is formal verification complement testing or vice versa?

Regards Ali