The title master makes it possible for you to customize your title slides to your liking without affecting the rest of your slides. A typical example is with header and footer options; you can choose not to show date and time, slide number, or footer text on title slides, to keep those cleaner. (You'll see how to do this in the practice.)
You can also customize the title master itself, to make it stand out even more, with a different font color, for example; or a piece of art that you want only the title slides to have.
Customizing the title master is especially beneficial if you plan to use the title slide multiple times in your show. For instance, you could use title slides to introduce new sections in the show. All the customizations you've made to the title master show up on each title slide.