A SIP IP phone (or an analog phone + a SIP ATA) can be connected to your LAN, e.g. to an available port on your router. It could access SIP services (Callcentric,
VoIP.ms, many others) directly and wouldn't need the OBi at all. However, you can also configure the system so the OBi acts as a VoIP bridge, making its services available to the IP phone. IP phones cannot directly access Google Voice or OBiTALK, so using the OBi to relay calls is useful. Most IP phones cannot connect directly to a landline; an OBi would allow use with landline calls.
I hope that the above is sufficiently clear. If not, please describe your proposed setup in detail.