Why does Paul use the human body as a comparison in 1 Corinthians 12:12?
The comparison illustrates unity and diversity in the Body of Christ: one body, many members. Just as a human body is one though it has many parts, believers, while never losing individuality, are brought into Christ and become members of His one composite Body.
Les explains that in 1 Corinthians 12:12 Paul turns to the human body as a comparison. The human body is one, yet it has many members, and all those many members still constitute one body; likewise is Christ, or the Body of Christ. Every believer has been brought into that Body and become a member of it. Les emphasizes that we never lose our individuality, but we are all part of the composite. The next verse shows how this happens: by one Spirit—the Holy Spirit—believers are baptized into the Body of Christ. Les calls this the only true baptism of the Holy Spirit for today, not water baptism, but a work of the Holy Spirit that at the moment of salvation places us into the Body of Christ.
For as the body is one, and hath many members, and all the members of that one body, being many, are one body: so also is Christ.