The default display is primarily a display to store settings, and isn't actually used anywhere within the Views system. It is possible for external programs to use the default display, but if they do they will (hopefully) tell you which display they will be working with. The default display is also a convenient display to use to embed into your site using PHP snippets; this is useful, for example, in node content, but this is something that should generally only be done by administrators.

In general, you probably want to add either a <a href="topic:views/display-page">page display</a> or a <a href="topic:views/display-block">block display</a>.