When you have a lot of projects, there are some that are just more important than others. They might be your pet projects, or the ones that are most important, or the ones that you want to a keep a more careful eye on. This is where project favourites help out.
When you make a project a favourite, it is shown at the top of the portfolio and additional metrics are shown in the portfolio page. Now not only can you see your favourite projects first, but you get some insight in to their performance without even needing to go in to the project detail. When you're done, you can remove the project from your favourite list.