What do the prostate and the bulbourethral gland contribute to semen?

Volume & enhancement. Bulbourethral or cowper's gland produces pre-ejaulate to lubricate & pre-clean urethra ahead of sperm containing ejaculate. Also alkalinizes urethra plus neutralizes and eliminates any residual urine. Prostate produces approx 25% of seminal fluid, also alkalinizes semen and tends to protect sperm from vaginal acidity and improve the odds that sperm will reach and impregnate an ovum.
Semen components. The prostate contributes ~ 1/2 of the fluid in semen with important components/factors such as zinc and certain enzymes. Fructose comes from the seminal vesicles. The bulbourethral glands offer lubricating fluid.