The app description is obviously there to give potential customers some insight into exactly what your app is and what it does. But, from your point of view as a developer, the app description is your chance to pitch. You have to sell the idea. You have to tell people why they should download your app instead of another.
You know that anyone reading the description has already found your app based on a search they’ve done. The name and screenshots were appealing enough for them to take the time to read the description. You’ve got your foot in the door, now you have to close the sale.