This is almost an ‘insider secret’ because many people don’t even notice these pink card readers at stations around London. However, they could actually save you money as you travel.
Working out fares is actually pretty complicated. You can start and end at the same station but take different routes to get there and it will cost you different amounts. Basically, if you go through zone 1, you are charged more. If you go AROUND zone 1, you are charged less.
Tapping in at the pink readers if you are doing a detour around the central zones means that the smart card will charge you less for the journey as it picks up that you went around and not through. More info here: Pink card readers.
We like this Londonist video explaining it in a bit more detail: Londonist Pink Card Readers.