Could you tell me what is the function of pre-ejaculate? why does it happen?

Lubricant +: It appears pre-ejaculate serves to neutralize the acidic environment of the urethra(from urine) and possible the acidity of the vagina. This is good for sperm. It also appears to play a role as a lubricant too.

Here are some ...: Pre-cum, based on what could be conjectured, is meant to prepare an favorable alkaline environ along the urethra for sperms to pass through and as well, provide physiological lubrication of the introitus for smoother vaginal penetration. It naturally excrets from bulbourethral glands (Cowper's glands) upon sex arousal.