Too Long; Didn't Read
CSS is used for describing the presentation of a document written in a markup language. If there is a child element it should be defined completely separate from the parent. Less and Sass are CSS pre-processors which extend CSS language in valuable ways. Nesting makes the code easy to read, extend, and maintain. It is an option which has same interpretation in less and sass so the next explanation is universal for both of them. Nesting must be used for pseudo-classes because it is a must. It's ultra-intuitive - just put ampersand next to pseudo-class.