Combat in Genshin Impact is element-based. You can mix two elements together to deal bonus damage or create a bonus effect, like freezing a target inflicted by water. Because of these combat considerations, you want to make sure you diversify your team with multiple elements. By swapping between your characters and using their abilities, you can command combat with elemental explosions.
But there are also reasons to keep numerous characters of the same element on your team. The Elemental Resonance system gives you a bonus based on doubling up on an element or diversifying. If you have four separate elements, you’ll get a bonus to your elemental resistance. But if you have two Pyro characters, you’ll gain a constant 25% damage buff.
Each elemental pairing comes with a different effect, so experiment and find out which combo works best.