The environment turn is made up of 3 parts: 1) Start of turn, where "start of turn" effects occur; 2) Play a card from the deck; where you would play a card from the deck (unless some other effect prevented that from being done this turn) and if it had a "when this is put in play" effect you would do that; and 3) End of turn, where any "end of turn" effects would occur.
Volcanic Eruption would not typically deal that 7 damage on the same turn it is played during the "Play" phase; you would have a chance to destroy it before the next "Start of Environment turn".
This is the crucial point - you have a chance to avert disaster if you choose to, often at the expense of one or more of your heroes taking a shot at the bad guy. Or, if it also harms the villain you may decide to let it go, let it tag everyone AND have more hero action to also add on the villain. So many strategic options . . .
In this case think of it as a warning temblor and a belch of smoke and ash - "She's going to blow!" - Does Legacy divert attention from Baron Blade to drop a massive boulder in the opening? Does Absolute Zero chill the magma into rock instead of taking out a few minions? Or do you double down on the villain, knock him out and jet to safety at the last second?
Ooo! The tension!