A photographer friend of mine put out the question on Facebook, do you caption all your photos? He does, but he finds that the task can be tedious.
Here's my answer.
I love captioning my photos on Facebook and Flickr. I find that a lot of people don't caption photos on Flickr, at least. Descriptions provide food for search engines bringing more searches to one's photo.
I also find descriptions are a means of self expression. I like to write in my blog, but not that many people read my blog. Breaking up my writing into various photo descriptions on Flickr, rather than writing to this blog. My photos on Flickr seems to bring a lot more readers.
Like Facebook, Flickr allows fairly long descriptions. There's the title / caption and then more space for a description below.
I have not been captioning photos that I don't post. That would be tedious. Some of these are close to being duplicates. Others are ones I don't think are worth posting, tho I often do change my mind given another look at the photo.
My photos are kept in folders on my computer under the label of the year the photo was taken.
The photos in my computer are not captioned, but I remember the circumstances for most of them. Also, a lot of those photos are taken around the time of the ones that I do post and label. They are usually photos from similar contexts.