I think when your party wants to fight very powerful enemies that all of them have magic weapons.
Who would attack a demon when you are only equipped with regular swords?
Just to be sure: DnD 5E makes only a difference between magic and non magic, right?
DnD 3E had resistences like 5/cold iron, 10/magic and so on.
In BG you need a macig weapon+1 for many enemies, some require+2, +3 is enough for almost everything and +4 was needed for the most powerful ones. (Kangaxx, Demigorgon, . . .)
For the most part, but there are exceptions. For example, a werewolf is immune to nonmagic attacks unless they are made with a silver weapon.
In 5e, most things are 1/2 damage or immune to damage (the feat Heavy Armor Master is -3 damage while using heavy armor).