I think the idea is not to do overtime. The idea is to use some of the slack you have because you finish your normal work early to consider process/architectural improvements or similar. Some people here consider it so obvious, that they even say it is part of your normal work. Well, I would say this mindset is much better than having worker bees who don't ever question anything and you end up with large inefficiencies because those who would maybe be empowered to define such work (like managers) don't have enough detail insights to actually do this.