A railcard is a sunk cost to the traveller, so it encourages them to travel more than they would have done anyway (which is why they're aimed at discretionary leisure travel).
It's a win win situation for the TOC's because they also gain the £20 or so for the railcard itself.
I reached the age of 60 almost 3 months ago in October and one advantage of reaching 60 was to be eligible to purchase a Senior Railcard. It was my first ever railcard because way back when I was in my late teens and early twenties the current form of a Young Persons Railcard was not then available. In the 3 months that I have had a railcard I have used it, or am intending to use it, in the following ways:
1.
Local journeys into my nearest city for leisure, shopping etc. where the return fare with railcard is often less than £5. Previously I would probably have used my car or even the bus for such journeys.
2.
Longer journeys, up to about 50 miles, for days out using Off Peak Day Returns. Fares of up £20 or so with a railcard are quite acceptable to do regularly, but I wouldn't do anywhere near so many of these journeys if I was paying the full price of perhaps £30+.
3.
Medium distance journeys to or from short break holiday destinations or to stay with friends/relatives. Mainly journeys of between 50-100 miles. Before I used to look for the cheapest possible fares by using advance purchase, splitting, longer slower routes or even quietly using fare anomalies and loopholes! I know that these are still available at an even cheaper rate with my railcard but I am enjoying the flexibility of walk up Off Peak Returns at a more reasonable railcard price.
4.
It's now really only long distance fares, where I will try to get the cheapest fare by using advance tickets or splitting. Here now I also have the advantage of the railcard reductions. Therefore this is the only type of journey where I am spending less in rail fares than I was before.
In all the other cases 1-3, I am making journeys I wouldn't have already been making or in many cases spending a little more than I would have done by making use of more flexible and convenient tickets at a reduced price rather than going for the cheapest, but often inconvenient option.