Many companies have websites and they want to have their information in Salesforce to connect to their website pages. It is important to have the websites accessed by as many people as possible, then it would be beneficial for exposure when people use a search engine to find something related to what the company is selling. If a developer created a page in Visualforce of the Salesforce program, then they may have a custom controller to navigate it to an external website which could be their own site or a backlink.
To access it, the command button can be pressed. The best recommended way to test to make sure this works is to get URLU on the rest of the action method and System.assert equals to compare the resulting URL.