I usually haven’t filled in the description field on my postcards, but I can see how it would be useful, both for myself and for others who might be looking for additional information - particularly on art cards.
Do you use them? For most cards or only certain types (art being one that comes immediately to mind)?
I’ve only used the description field for one purpose–to indicate that someone has sent me a greeting card rather than a postcard. I know in some cultures, there’s no distinction between the two, but I like to be clear about this. I think it also helps on the off chance that a random person is browsing my wall wondering if they can get a particular postcard. It will indicate to them that it’s a greeting card instead so they don’t go off on a wild goose chase trying to find a postcard that doesn’t exist.
I want to know what is represented on a postcard and I guess others will too. Therefore I always add the relevant information as image description.
For the postcard I sent, if the recipient is not filling in the image description, I add the information in the comments to the postcard.
I am sorry this is not common practice as I often want to know what is represented on cards others received but nothing is indicated.
I do as @syaffolee and indicate folded cards in the description, as well as cards sent from another country than the ID says.
I also write the description that is printed on the back of the card into the description, as I like to be precise there. 
When there’s a lot of text, I write it in the comments, like @PasoDoble .
Yes, I use them for postcards with birds to indicate bird species and for art to indicate author name 
Very good idea with adding descriptions in comments to our sent postcards. I always describe received cards and will do it your way with sent ones. Thanks for your suggestion.
I’ve been adding where the card originated, like which box set.
I’m not really using the description field, because I’m waaaay behind on scanning received cards. I would complete it on every card I send if I could, but I can’t. 
I never even thought about it! 
And barely noticed that such option exists!
Occasionally I used comments if I had to say something about the card…
It makes total sense to add a description but it never occurred to me before right this moment! 
Sometimes I do, if the card has an interesting information about it and the sender tells me about it or when I feel like it can be useful. But most of the times I don’t. It’s a good idea though.
I used to do it quite a bit, but then I got lazy… I might try to do it more often in the future!