The description only appears on the home pages of spaces, and files that have been added to the community. Adding other pages to the space navigation or anywhere else in the community does not have the description field.
You can however set those pages as templates, and add template descriptions to those pages. On pages there is an option "Set as template", when you check that checkbox you can add in a template title, and description.
I have created a feature request to our production team on potentially adding this enhancement to the digital workplaces to allow pages to have descriptions. The reference ID for this request is: ID181123622.
Let me know if you have further questions on this.