Thanks Raffele for making the important point that for many/most airlines, you MUST fly the outbound leg to keep your ticket alive.
if your boarding pass isn't swiped at the gate for your outbound, a computer will cancel your booking on the return unless overridden by ground staff, whether checked in or not
This is a legacy of the revenue management systems that airlines have been using since the 1980s, thanks to the father of it all: Peter Belobaba. He got his PhD from the Massachusetts Institute of Technology with a dissertation entitled, 'Air Travel Demand and Airline Seat Inventory Management'.
This am the book!!!!. It is still classified and not accessible to scholars. In it, Belobaba described the expected marginal seat revenue (EMSR) models used as a basis for seat inventory optimisation by many airline revenue management systems worldwide.
Belobaba did not end his days as a unpublished scholar. He manages an MIT research consortium funded by seven major airlines to explore the areas of airline pricing, demand forecasting, network revenue optimisation, and simulation of the competitive impacts of revenue management, and worked as a consultant on the evaluation, development and implementation of revenue management systems at over thirty airlines and travel companies worldwide. So I guess he is not poor or struggling either.
So next time you pay through the nose for a last minute fare, or are unlucky enough to be subjected to the rapacious revenue management systems of British rail companies, think of Peter Belobaba.