When it was first released, the browser plugin was free so it was incorporated into a lot of web browsers. This meant that loading times for games and other software that needed Flash to run were shorter.
There are a number of reasons that Flash Player was so popular, one being that flash files were very small. It works by running content from SWF files, which is an Adobe specific file format for multimedia and animations. The software is a runtime, or a system that describes the library that coding language runs on. At one point, the software was required to run most interactive applications on the internet.
In its prime, Flash Player was a must-have. Outdated, unsafe software has met its end-of-life Software to view multimedia has long been surpassed by competitors