Why is removing presentation-oriented markup from one's html document considered to be a best practice?

Answer

Its for the purpose of maintenance and readability

Explanation

A presentation-oriented markup web application generates interactive web pages containing various types of markup language.

Markup is a language designed for the purposes of processing definition and presentation. It specifies code for formatting, both the layout and style, within a text file. Markup describes the structure while the styling is how it looks. it uses the code to specify the formatting which are called tags.