It comes from quantum mechanics where particles travel all possible routes and when making calculations you need to account for all possible variations, because they affect each other.

When particle is measured, all variations disappear except one and that one continues thereafter without the affects of others.

Multiword model dictates that the other variations don’t disappear. They just disconnect from our reality and continue elsewhere.

Currently multiworld model is popular. Imho it 's the only one that properly explains all quantum phenomena.

Although there is a good argument wether multiworld is proper science.