WordPress is a database powered platform which generates content dynamically. The more content and plugins you have the more database requests are executed. This can slow down your sites performance, especially when your database is located on a different server.
A cache plugin like Borlabs Cache saves the dynamically generated content as a static file on your servers storage. When the page is requested, this static file is loaded and send to the visitor, which is dramatically faster than the regular database requests. But that is not all. Your pages contain a lot of unnecessary whitespace or HTML-Comments, which only increases the overall size of the page, but are not important for your e.g. layout or design. Borlabs Cache removes all of them and uses GZIP to compress your page, which saves your and your visitors traffic.
These are just some examples, what Borlabs Cache does to increase your sites performance.