The placeholder description "enter image description here" implies that users should always add their own descriptions — why should I enter anything instead of leaving the field as blank?
why should I enter anything instead of leaving the field as blank?
You should do this because you want your question to be accessible and readable for everyone, including blind users, users with browser configurations that block direct loading of images, or users in environments where Imgur is blocked.
I think its not a matter of why - its also a matter of when and what. (and it makes it easier for some folks).
Alt text serves a few purposes. It lets folks with disabilities know what an image is. And it lets you add information about an image that might not fit quite well in a post without breaking flow.
Now I could tell you its an LFM66 but most of the time, its just used as an example of a strange, single purpose machine. Without the image, you'd not really know what is is.
The hint lets me add a description so someone who can't see what it is can read or be read out a description of it.
It shows up in the markup like this
So a screenreader or other accessibility tool can see it.
On my favourite text mode browser (w3m), the page renders something like