I'd like to see the Catacombs of Paris in September 2017. It appears that tickets can be purchased in advance from http://b12-gat.apps.paris.fr/accueil.aspx ... is the demand such that purchasing tickets in advance is necessary if you want to see it?
When we visited the catacombs in May of 2015, we went early in the morning, before they opened, and still had to wait for about an hour to enter. Do the advance tickets give you a guaranteed time slot to visit? If they just prevent you from having to pay once you're in the door, I wouldn't bother unless you receive a significant discount. Paying is not the problem, demand is. There is plenty of time to pay as you wait for another person to exit. (The catacombs are visitor-restricted; you can only enter when another person exits, unless you arrive at the very beginning of the day before the catacombs reach visitor capacity.)