(Hopefully I've catagorized this right....)  

What functionality is missing? What is unsatisfying with the current situation?

You can search images via their tags or names, but not their ID in the system of WA itself. I have images in old articles using the old system of [IMG:######] that I'm unsure if they've been deleted, or the system doesn't know how to find them.    

How does this feature request address the current situation?

Being able to search your image gallery by the ID number could be useful in situations like this where you are unsure of what the image title could be. This would only search within your own galleries, to avoid people using other people's items.  

What are other uses for this feature request?

If unsure if an image is deleted or not, you can search the number code; if it doesn't show up in your search of your gallery, then you know for sure you've deleted it.