The hCG shot is usually give shortly before ovulation for the following reasons:

• It helps prepare the uterine lining for implantation by making it more spongy (and easier for a little bean to stick)
• It helps lesser mature follicles to catch up with the dominant one
• It forces mature follicles (15mm and bigger) to be released
• It make you ovulate withing 36-40 hours after the shot

The hCG shot is given during fertility treatment to pinpoint the exact day of ovulation and prepare you for procedures like intra uterine insemination IUI. Even if you ovulate on your own, you can gear up to ovulation and not actually do it, which would ruin the timing if you're doing an IUI. Second, if you have egg quality issues, the more mature follicles you release, the better your chances that one will stick.