I think this is a compromise for the Devs.
Instead of giving all accounts with Season 2 unlocked a huge Hero XP advantage over other players, Season 2 is roughly the same.
Season 2 gives players an advantage for loot tickets.
Similar to my preferred auto play farming being 1.7-7 for most recruits, 1* / 2* heroes and 1* / 2* troops per auto play hour.
But if 3.5 days out of each month, Season 2 is substantially better, people will try to unlock Season 2, upgrade their hero teams, buy VIP, buy loot tickets and by world energy. Generating IAP for the game studio.
Very similar to Challenge events lasting 3.5 days.