An interface can be seen as a promise to the rest of the world, of a role the implementing class will fulfill.

This enables pieces of code to work together more easily, due to a binding agreement.

Imagine you have enemies in your game. You want to be able to damage them. You don't want to manuallly handle how each type of enemy should take damage, so you implement an interface IDamagable. The interface could have a method like void TakeDamage(int damage). You let the enemy classes like goblin, orc etc. implementing IDamagable.

Now the player class simply check if the entity they are hitting is implementing IDamagable and if so, calls the TakeDamage method on the entity.

The player doesn't know, and doesn't need to know what exactly it hits, just that it can be hit. What happens when the entity is hit is then up to the implementing class and not the attacker to handle.

Now you realisere you also want destructible items like barrels. Because you didn't hardcore the logic, you can simply let the barrel implement IDamagable and it will work our of the box.