I say, with Allah's guidance: The obligation is the engagement of the conscience, and it has a real reason and an apparent reason. As for the real reason, it is the eternal command of Allah, Glorified and Exalted be He, because the one who obliges the rulings is Allah alone. However, since His obligation is unseen to us and we cannot perceive it, He made apparent metaphorical reasons for us to ease the matter, which are the times; as evidenced by the renewal of the obligation with their renewal. As for the apparent reason, it is the time; as evidenced by its attribution to it, which is the indication of causality: like the punishment for adultery and the expiation for an oath. See: Al-Maraqi p.172, and Al-Tahawi's commentary p.173, and Allah knows best.