Networking may indeed not be necessary if you are white, middle class, university educated and your offspring go to a school with a history of getting their charges into Oxbridge. If you're none of the above (or even can't 'tick' just one of those 'pre-qualifiers') then you're on your own to face a subtle and sophisticated admissions procedure about which you know little.
Networking shouldn't be necessary (or desirable?) if, as Boffin suggests, there was a logical selection procedure which was fair to the truly able irrespective of their background.
Bear in mind there was a Russell Group hoo-ha fairly recently over the 'correct' A-levels to take if candidates were to stand a chance of gaining a place at one of these better universities. There are many parents out there whose only guidance on 'A' level subject choice comes from the school who want to shunt a fair number of pupils into hopeless Media Studies-type 'soft' subjects because it improves the school's strike rate in the league tables. Educated and discriminating parents already know these subjects are hopeless for Oxbridge, but there are many who don't have the same explicit knowledge (or ability) to de-code the whole 'A' level/Oxbridge entry puzzle.
If the school is of no help to potential Oxbridge candidates then where do you turn for help? Networking levels the playing field for the uninitiated.
Yellowtip: The Cambridge 'mock' interview. I took DC to Cambridge for the interview when the interview process was in full swing out of term. It can be a hugely anxious time for candidates and certainly the atmosphere in the corridors etc where the interviews took place was a bit like the dentist's waiting room when awaiting root canal work. There were a lot of extremely anxious young people there - most of them ashen-faced.
My DC was sixteen at the time of interview and after the event commented that the stressful nature of the preparation did help on the day. No doubt supremely confident candidates from privileged backgrounds will have had plenty of 'high table'-type discussions (where you're forced to think on your feet, make sense, and defend your position etc) so might even enjoy a bit of verbal/intellectual 'fencing' with the admissions tutors. If however your background is less exalted then you might find the interview a bit of a shock to the system, clam up, or blabber on making no sense at all.....for these candidates intense/semi-realistic preparation will be vital.
Oxford selection process compared to Cambridge. A very good friend is an admissions tutor at a middle ranking Oxford college. When comparing notes/experiences with them there was little substantive overall difference, with similar outcomes. If anything Oxford more likely to select its traditional demographic compared to Cambridge. (I think recent independent studies back this up?)