Two plus two is four. Minus one that's three, quick maths!

Let's count how much gold you need to spend if you purchase everything separately.
— 6-weekend brawl contracts, 250 gold for each: 1,500 gold
— Battle career contract: the equivalent of 400 gold
So, if you want to get the most out of the season, you need to spend 1900 gold, and that's a lot!
With a Season Pass you'll get the same, but much cheaper. Who doesn’t like discounts? Moreover, pleasant bonuses are included.