By definition, Wikisource is a library containing only texts that have already been published somewhere. Most of them are imported from Html pages of various sites, the rest is probably 20% is the import from Pdf scans, and a small percentage is imported from other formats. There are no other sources of content for Wikisource.
That is, users import most Wikisource content from htmls, each time manually converting them to wiki-format. This spends valuable time, which can be used more productively for the Project. In addition, usually the text from html is copied without formatting the styles, the restoration of which manually also takes a long time.
Installing this extension would be very helpful. In addition, by the description, it can to upload illustrations, many books have tens and hundreds of them, that will also save a lot of time.