>Why bother retaining semen in the body?

Retention of semen in the body generates progressively increasing physical and mental strengths.
The body also starts getting smoother and shinier.
It increases the processing power of the mind, your ability to concerntrate will increase, memory improves.

When a man ejaculates, the first thing it decreases is the protective energy field that surrounds not just the body but also inside the body (it starts at the heart, goes all over the body, until the outermost layer of the skin, and then extends a few feed beyond the body.
This energy field (often called one's "Aura") cannot be seen by the naked eye, but it is what creates a certain kind of "glow" and "luster" around the body (by reflecting light).

The protective energy field and the shine it gets are primarily from long stored semen.
A weak protective layer also gets "thinner" and develops "pores" in it thus making it looke "darker" as it can't reflect back all the light, so the brightness of the body stars disappearing.