Opinion seems to be divided on this and I don't know what to make of it. I don't think the semen itself plays a big part, and it's more physiological than anything.
I've always been a little skeptical of it and I wouldn't be surprised if it is just placebo in the end. I do semen retention by default if I'm not pmo'ing, but its generally never been a big deal to me. If it helps others though, I say more power to them.
I've heard that the semen released during wet dreams is different to usual, and I've personally experienced that myself.