Alex only needed Bladedance to explain that one part for the rest of his confusion to get sorted out all on its own.
Qi at a higher cultivation realm was a scarce resource. Even for Divinities, they needed a lot of Qi in order to properly cultivate and breakthrough. As a result, they either had to gather a lot of resources to acquire the Qi from, or they could just have heaven do it for them.
After each completion of a tribulation, the Qi left behind by the heaven would be more potent than gathering treasures. As a result, most people felt a need to call upon Heaven when they needed Qi to cultivate.
It was a risky method of cultivating, as each tribulation would be stronger than the last. And since a tribulation came every 10 thousand years, if you ended up calling a particularly strong tribulation lightning in that time and failed to improve, you would end up dying.
