As another person already stated it simply speeds the game up, your fire rate, the travel speed for you and the enemy, etc., just makes the whole game faster. I played 2 different accounts for a while and used it on one but not the other and there's little difference, it only costs coins to level up but I would recommend not using it and save the coins for leveling up your mobs, cannons and such.

I've never seen that locked before, but I play on the android version for my phone, under it is where you're able to complete tasks to earn bonuses such as extra cards, shields, stars, etc. Maybe check the store to see if your version requires you to buy that feature, there's no real need to spend money to play but since I play it so often and enjoy it I did spend a bit to unlock the triple cannon and get rid of ads.