A: In the strictest sense, it's not a matter of 'need', but how we choose to be in the world. I think we can all admit there is more to life and the Universe than what we can perceive with our senses. There is something more that lies both beyond and within each of us. "God" and "Spirit" are the names we give to the connecting creative Presence in the Universe. It is what we are at our core. It goes by many other names...Atman, One Mind, Tao, All That Is, just to name a few.
In Unity we also refer to it as our Christ Nature or Christ Consciousness. Our spiritual and life practice is to transform or align our human personality with our Christ Nature. The human personality tends to live from fear and separation, which leads to poor life choices. When we live from our Christ Nature, our choices tend to be more loving and compassionate. This is not to say we couldn't be loving and compassionate without aligning with a concept of God, but it's also about a deeper experience of ourselves... our Soul.
If we didn't have "God" or "Spirit" we would probably still be talking about the deepest part of our self, about the mysterious connectiveness of the Universe, and everything in between. And ultimately, all words, names, and concepts fail to accurately describe what can only be experienced without words.