A draft should not live on someone else's server
Markdown editors get used for meeting notes, internal documentation drafts and text that is not ready for anyone yet. Most online editors store that content in their backend. Here the document lives in your tab's memory: close without downloading and it is gone, and that is precisely the guarantee.
What syntax the preview handles
Headings, bold and italic, ordered and unordered lists, links, inline and fenced code, blockquotes and tables. The toolbar inserts the syntax for you, so you do not have to remember whether a table needs colons or dashes.
Markdown versus a word processor
The point of Markdown is that the file stays plain text: it versions cleanly in Git, it reads without any application installed, and it carries none of the invisible formatting that makes a .docx break when opened on another machine. That is why READMEs, wikis and site generators use it.