The general idea is to provide the user with the main expected result as early as possible.
If I have to fill a description before the upload starts as a prerequisite I'll see this as a barrier. However, if the upload starts and I'm able to provide a description meanwhile, then I'll be happier to do so and probably be willing to invest more time on it since the upload is going on.
Uploading a video to YouTube is an example. while the video is uploading you have the opportunity to edit descriptions or add another videos.
The system can even enforce that a description is provided before the content is publicly available, but that does not conflict with the idea of not blocking the upload from starting. I made a quick sketch to illustrate a possible workflow where asking for a description is postponed as much as possible:
Pau