The farmer class has a skill that lets me slack off for 1 hour at the cost of 2 sp. What is the purpose of this skill and when should I use it?
Horseplay, as you've noted, simply advances the game clock by one hour. While it seems useless on the surface (and, according to some, it mostly is, for reasons explained below), it's not entirely without use.
For example, if you're gathering in the dungeon late at night, advancing the clock to midnight will reset your harvesting allowances. Most players seem to usually recommend Double Crop for harvesting runs instead, but one small benefit that Horseplay offers over Double Crop is that it only takes a single skill point to invest in, and has no significant TP cost, which can be useful in the early game.
Another minor use is as a way to manipulate the in-game clock for specific dungeon events that only occur at specific times of day.