The SONOS BOOST is a wired to wireless bridge. It sits next to and plugs into your router.
BOOST provides enterprise-grade wireless performance for your Sonos system so you get unparalleled reliability - even in homes that are known to have wireless connectivity problems.
It is needed if you are using any of the SONOS speakers wirelessly. In other words, unless you plan on physically plugging your SONOS speakers into your local area network, you will need a SONOS Bridge for your SONOS speakers to work with WIFI.