The general concept is called Spinors, Quaternions are just one representation. Here's a great video on them. In physics they're using them because they're necessary (video explains), in computer graphics we're using them because they're algorithmically convenient, very cheap to compute and ignore that whole half-spin thing. It's one of those instances where it's cheaper to compute useless information and then throw it away as opposed to avoiding to compute it.

They're also absolutely impossible to deal with when authoring stuff, as in rotating things in Blender, it's just a representation on the backend. Quaternions would avoid gimbal lock but when authoring you really rather deal with that than a 4-dimensional hypersphere.