Why: having spoken thus, Krishna, the great Lord of yoga, then revealed to Arjuna his supreme, divine form, moving from description into the actual, direct experience itself.

In life: at some point, description has to give way to actual, direct encounter, you eventually have to stop hearing about something and actually experience it yourself.

Teaching: notice when you've spent long enough hearing descriptions of something important and are actually ready to move into direct encounter with it yourself, rather than continuing to only gather more secondhand description.