The rules say that creatures in a heavily obscured area "effectively suffer from the blinded condition".
The blinded condition states:
Attacks rolls against the creature have advantage, and the creature's attack rolls have disadvantage.
So when the Darkness spell (to use one example) is cast, combatants gain advantage and disadvantage when attacking others who are also inside the area, which cancel out.
Since they then cancel each other out and attacks are made normally, what really changes?
Best Answer
It levels the playing field
Casting darkness will cancel out both advantage and disadvantage, due to the way they stack.
If your opponent has advantage and you have disadvantage, then cancelling both will be good for you, and bad for them.
It benefits those who can see through magical darkness
Some characters (eg, Warlocks with Devil's Sight) can see normally in magical darkness.
So, their opponent will be blinded, but they won't.