Why do Helios and Boreas agree to have the
contest?

A dispute of power.

Explanation:

Helios and Boreas agreed to a competition to prove who was the most powerful between the two. Boreas, The Northern Wind, tried to blow the man's clothing off with all of his might. The more he tried, the tighter and more snug the garment became. Boreas called to Helios to prove his strength. Helios, The Bright Sun, shone his warm rays of light upon the man, and slowly he took off his cloak.

I love Greek mythology, so I hope this helps! :)

there were a lot of changes in wars like that and thing evolved

explanation:

a.) it provides a clue to solve the mystery.