Basically because in languages where everything is immutable, 'setting' a value nested in a datastructure is a big hassle. Lenses give you a means to effectivel

Basically because in languages where everything is immutable, 'setting' a value nested in a datastructure is a big hassle. Lenses give you a means to effectively do foo.bar.x = "baz" succinctly (and a few other bits besides).