if you have for example 2 fire heroes in you attack team, you do double damage(depends on heroes’ attack power) when you hit an enemy with red gems. Especially with countering (like red againsts green). It gives the team more outplay potential, you are more likely to kill an enemy hero before his mana bar goes full.
Also if both of your fire heroes have same mana speed, they can use both of their abilities at the same turn, give your enemy heroes no time to respond.
I have 2 fire heroes, 2 ice heroes and 1 dark hero in my attack team, my strategy is only matching red and blue gems, then purple gems. when there are no red or blue or purple gem on the board that can match, I try to create yellow and green diamond to clear the board(its a lot easier to create diamond at this time, and likely follows with multiple combos).
of course this strategy has a huge downside, one or more color gems does no damage. Sometimes you can even see an auto-lost before you even start.
But I’d like to take the chance and continue playing this way. Maybe I’ll change my opinion when I reach the late game…