Timothy did not co-author this letter (Philippians 1:3 is clear on that), yet Paul includes Timothy because Timothy worked alongside and supported Paul in his work (Philippians 2:19–30). We need this in ministry (Exodus 18:18). Ministry is not a one-man show. Paul needs the support and encouragement of Timothy (and others), and so does every preacher of the gospel.
1 Paul et Timothée, serviteurs de Jésus-Christ, à tous les saints en Jésus-Christ qui sont à Philippes, aux évêques et aux diacres: