I do the same. We have a rotation of buffs going, so it's simply easier to purchase them ahead of time and then activate them each night without having to head back to buy them if I or an officer isn't heading that way.

As for after the 24 hours is up, the buff disappears following its use and will then need to be repurchased, whether it completes the entire 24 hour buff or is deactivated in order to activate a different one.