My very long standing point about Advance tickets is that it's fairly clear that train companies do use them to sell a specific number of seats and vary how many they plan to leave for walk-up custom, factoring an estimation of the revenue from walk-ups that makes it into that company from RSP. This is a rather removed process from what some people think about the way in which advances are used is, which is to imagine none are sold and apply cheaper tickets to those seats.
It's very clear from recent franchise agreements and ITTs that DfT expects Advance tickets to be used to smooth demand across train services. With DfT putting a clause in franchise agreements that capacity planning provides passengers with a reasonable expectation of a seat on off-peak services and within 20 minutes of boarding a peak service, TOCs are going to be using Advances over walk on tickets more.
Tale the TPE ITT as an example (emphasis mine):
"Forecast loadings or Peak standing in excess of these requirements of
up to a maximum of 10 passengers per carriage will nevertheless be
regarded as meeting the requirement if, and only if, the Bidder
demonstrates to the Departments satisfaction that it will, in practice, be
possible to
avoid those excess loadings or Peak standing by appropriate
yield management techniques whose effects have not been modelled in
the Bidders loading forecasts (i.e. reducing availability of advance
purchase fares on the overcrowded train and increasing availability on
other services which have spare capacity and which operate between the
same two points within an hour of the overcrowded service in question)."
