In most countries you are taxed note the more you earn, so two people earning 60k is MORE than 1 person earning 120k.
Taxes are paid in brackets and having two independent incomes makes each one fill their own lower bracket before going for the upper ones.
It does make a difference, in Spain for example nowadays even married couples fill taxes separately because it’s just not worth it tax wise to join incomes.
88 -> HH -> Hail You know who.