The zone fairs aren't really distance based. The goal is to disincentivise core travel especially in peak, so it's actually cheaper (but of course slower) to skirt around the core when crossing London.

Suppose you enter at Upminster and leave at Moor Park, those are both in Zone 6. But by default the system will conclude that you probably passed through the core (Zone 1) since that's the obvious route and charge you for a journey using zones 1, 2, 3, 4, 5, and 6.

If you're on a budget and no time constraint, you can travel to Barking, get out, touch the Pink validator, board a train to Gospel Oak, touch another Pink validator, and now finish your journey to Moor Park, and since you've avoided the core you're charged a significantly lower fare (but this takes much longer).

Before Oyster this couldn't work, you weren't allowed to travel on the cheap fare via the core, but there was no way to detect that you'd done it - the tickets don't know where they are, so people routinely did and may not even have realised what they were doing wasn't legal. But because Oyster (and thus also contactless fare system) knows the journey you made, it can conclude that (unless it saw you at the Pink validator on a different route) you went the obvious way and should be charged accordingly.

For most people this just made things slightly fairer. For a handful of people who liked cheap weird routes or are in no hurry it added a step (touching the pink validator).