When you are wielding two daggers and another creature hits you with a melee attack, you can use your reaction to add your proficiency bonus to your AC for that attack, potentially causing the attack to miss you.
When you attack the same target with two different daggers wielded in different hands, and hit with both in the same round, you can use your bonus action to do an additional die of damage to the target.
When throwing a dagger, you may throw it up to its maximum range without penalty.
