Les Sables d'Olonne
If there is a fast rail system, direct trains increase the number of tourists in a coastal resort. "Direct" means, that this train runs on the highspeed line first, and after that, reaches its destination on the old network, without any need for the customer, to change trains. This kind of operation is standard SNCF practice - as one example, all the TGV to Nice operate in this fashion.
Direct highspeed trains increase the number of tourists, their absence decreases this number in favour of other places. The honorable citizens of Les Sables d'Olonne concluded, that they wanted direct TGV service as well. Their lobbying effort was successful, and their direct train got support out of public funds. There was just a little problem: Les Sables d'Olonne is reached by a railway line, but not by the catenary, which is a suboptimum solution for an electric highspeed train.
The problem was solved by the TGV Vendée.