Not necessarily. With the appropriate Decklink card from Blackmagic Design, WAGS2 and CasparCG can key graphics over video internally. It can also be used as a standalone playout system.
If you do have a video switcher with downstream key capability, WAGS2 can output separate key and fill signals for your switcher. It is also possible to send a clean program feed to WAGS2 and have it handle the keying internally.