Why did people believe Osiris was great for them?

Because it was considered a symbol of life.

Explanation:

Plutarch (Ancient Greek philosopher) recounts how the Egyptians believed that Osiris introduced people to the trades, grain, barley, wine, and taught the ancient Egyptians how to cultivate fertile areas of the Nile River.

With all of these characteristics, the Osiris spirit identified itself with vegetation, fertility and the general power of nature. In the centuries after the creation of the Osiris legend, it was filled, but its essence remained the same. The Osiris legend is Egypt's most famous ancient story.