The practice of keyword stuffing is a black hat SEO practice. It’s used to artificially inflate the number of times a keyword is used on a given webpage. The reason some marketers choose to keyword stuff is because it’s intended to increase the webpage’s rank on search engines.
However, browsers are aware of the practice of keyword stuffing. They have built algorithms to identify when it is taking place, which penalise those doing so to discourage the practice. The Google Penguin algorithm is one example of an intervention on the part of a browser to reduce the instances of keyword stuffing.