The sun is trapped in the hold of the rotating celestial gears and is moving in and out of the radius like a needle on a record player. The stars also do this.
The reason for this North-South movement of the celestials is unknown. However, the seasons would not exist without it, and by extension, a diverse evolution of life driven by climate-induced adaption would probably have never occurred in history. We likely owe our existence to this phenomenon.
The question of why things are the way they are is immaterial. If it were any other way we wouldn't be here to talk about it.