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?