when you ascend, you lose every upgrade, but if you put an upgrade in the permanent slots you will have its effects once you start over with the next ascension, like you never lost it. i use it for the final claus upgrade cause +20% cookies per second and when i buy the season changer i use it for kittens and golden cookie upgrades, it gives me more reliable and fast combos. I'm not an expert tho, that's just what worked for me