Getting to a good university is only part of the battle and the real prize is the job afterwards. Having a big network is what helps with the latter.

Take law, even at Oxbridge only about 10% of students on that course at either university get into a training contract to become a solicitor. Its closer to 1% at normal universities.

Getting onto that training contract is knowing how to present yourself to the right contacts and go to the right events.

Many subjects are like this, especially for the top jobs.