A project portfolio view is a convenient way to save different subsets of projects in a portfolio, so you don't have to create multiple portfolios and look at each one separately. For example, in this portfolio, Jane has one selection of projects and John has another selection of projects. Without portfolio views, you would have to save 3 different portfolios: one with all the projects, one with Jane's projects, and one with John's projects.
With portfolio views, you can simply click on the view and the subset of projects associated with that view are displayed in the portfolio form.