Some of Markdown's main advantages are:
With rich text documents getting compatibility right to allow you to open it without breaking everything in different environments and on different programs is not easy. Not to mention may of the rich text editors tend to be resource pigs with well known quirks when editing text. Think of every time you had a word document wigg out when you change something that should be simple.
A markdown document is just simple text and can be expected to have a reasonable reproduction wherever you open it. If not converting it to the right format is much easier.
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In the upcoming versions of Nextcloud we will see the introduction of the text app. This gives you the advantages of both Rich text entry and saving in plain markdown text files in the backend.
Yes, tables would be good to have and are something we want to do. Sadly tables are badly standardized so we'll have to figure out which way to go.
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Help welcome! https://github.com/Nextcloud/text