Why did ranchers brand their cattle?

Answer 1

It's a way to tell if those animals are theirs if they are either stolen or if they are grazing with animals from other ranches. Each ranch has its own individual brand, and this way cowboys could easily tell which cattle belong to that ranch they're working for or not. They still do.

Answer 2

Branding !... They used to use a hot iron to burn an identifying mark into the flank of cattle.