For me, adding alt text is really important. Historically I didn't understand why it was important but I've since learned better - people accessing the site using screen readers or otherwise text-only should have the best experience that we can offer them, so if you're editing someone's work that has an image and the alt text is the default, it's very much appreciated if you fix that problem.
Images and infographics are really succinct ways to convey information to people who aren't visually impaired but you should be certain to cover the same information in the alt text - as much as you can. I generally combine explaining the image in the actual body of the post and adding whatever is missing to the alt text.
Let's look at an example that many of you may be familiar with.
This is the iconic image we've used for years to help users understand what is an answer and what is not, particularly regarding link-only answers. It's from Shog9's question Your answer is in another castle: when is an answer not an answer?
Currently the alt text for this reads:
A, NAA, A, NAA, VLQ
Well... this is... possibly useful to people who know what these letters mean but they don't actually describe the image so that someone who can't see it can benefit from it.
Now let's look at the version of the alt text I'm using in this question.
Image composed of five images explaining the variety of answer types through fruit. 1. a red apple, labeled "Answer"; 2. an orange, labeled "NOT Answer"; 3. an apple core, labeled "Partial Answer"; 4. a sign with a pictograph of an apple and an arrow pointing up, labeled "NOT answer" representing link-only answers; 5. a half rotted apple with worms in it, labeled "Low-Quality Answer".
Yes, this is a long description but it actually makes the image useful. Remember, "a picture is worth a thousand words", so sometimes, long descriptions may be necessary.
While my description above is long, it doesn't go into unnecessary detail. I don't describe the color of the apples or the arrow. I don't note which have leaves and which do not. They're irrelevant to the image. I could have chosen a set of images with green apples and made the same point.
And, this sort of description is necessary in Shog's post because he doesn't otherwise explain the image in the question. If the body had included a paragraph like:
These are our five different answer types. We want the whole apple. The orange is not an answer because we want an apple. The half-eaten apple partial answer is not really going to satisfy us. The pointer to where you can find apples is the equivalent of a link-only answer, which has the risks of link-rot described above. And... the rotten low-quality answer apple just makes us feel ill.
It would have been possible (though not necessary) to shorten the alt text and refer to the prior or following paragraph as explanatory.
An image composed of five images of fruit: a whole apple, an orange, an apple core, a sign pointing out where to find apples and a rotten apple with worms. Further explanation in text.
It's good to include any text in the image that is important, as in my long description I noted the labels for each image. Without that text, describing the fruit is less useful. Recently I was posting screenshots of my sock accounts "My Communities" section of the site switcher to illustrate how it works. Here's the image and alt text:
My site switcher showing Parenting - 83 rep, Computer Science Educators - 46 rep, Meta Stack Exchange - 3 rep, Android Enthusiasts - 1 rep, and Arqade - 1 rep
I explain the relevant parts of the image - which sites are listed and my reputation on those sites - but I don't mention the "edit" link or the "more Stack Exchange Communities" or "company blog" because it's not helping to illustrate my point.
Now, is this perfect? I'm still assuming that someone knows what a site switcher is... but I'm hoping that, in concert with the rest of the text in the question and answer, this does more than "enter image description here".