This option is a simple tool to calculate and update the portfolios a user has introduced into the system. A maximum of 10 portfolios can be created. Each portfolio has no limit in the number of stocks to contain. To learn more about this tool, you can consult this Help.
All the information related to the portfolios is recorded in your browser local storage. So, in order to maintain your portfolio data, you need to keep on using the same combination browser + computer. (The only purpose of storing the portfolio information in the browser is to be remembered for a personal future access).
The main advantages of using this kind of storage is to avoid entering the data each time you use this tool and avoid using an authentication system for it.