Why RTF is still around
RTF was Microsoft's pre-XML interchange format, dating to 1987. It survives because it's the lowest-common-denominator format that preserves basic styling: bold, italic, headings, lists, simple tables. US federal court filings, some state government workflows, legal billing software, and old WordPerfect ecosystems still produce RTF. The Mac's built-in TextEdit defaults to RTF.
What converts well
Headings, bold/italic/underline, ordered and unordered lists, simple tables, hyperlinks, embedded images (when present): all preserved cleanly. RTF is more constrained than .docx so there's less to lose. The conversion typically produces cleaner Markdown than .docx because RTF doesn't have the run-fragmentation problem that Word documents suffer from.
What's simplified
Custom font specifications, exotic colour markup, embedded OLE objects (rare in RTF anyway), revision marks, and form fields are simplified or dropped. None of that is content. If you also have PDFs of these documents, see PDF to Markdown for them.