There are basically two ways to get a solution on a forum like this.
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The most obvious way is to announce your problem or question to the forum, and if someone can help with your issue they will try to answer your question or be helpful in other ways.
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The second way is to search through solutions that are already posted to the forum and discover the information you need from what is already there.
Quantity-wise, most answers are found the second way because one solved forum thread can answer the same question for a thousand people (or however many). In this way, it is the more crucial solution to preserve.
The problem with screenshots is you cannot search them with text. Maybe five or ten years from now it will be possible, but as of today it is not. As someone mentioned, there is nothing wrong with sharing a screenshot to help illustrate an issue, but when it is text it is a terrible waste.
If you make a forum post with an unusual error message, but the error is a screenshot instead of text, chances are someone will still emerge with the solution if they know what it is. However, the crucial second method for finding a solution is destroyed, because that image won't come up in a search for the error message. So that solution fixes one computer instead of a thousand.
That is why people get so frustrated with the screenshots. It seems like a minor detail, but it's not--it really matters. It really is important. It can profoundly impact how useful a technical forum is able to be.
I think we all have seen hostility in various forums at one time or another, and to be clear I am not trying to say that kind of behavior is warranted or justified. Still, I can empathize with the frustration that must be felt when someone is spending their time and effort trying to make their forum a good and useful resource, and people are messing it up by taking a lazy shortcut.