Word documents are editable by design. That is useful while drafting, but risky when you are sending a final version. Different fonts, page sizes, margins, and office apps can make a document reflow when the recipient opens it.
PDF locks the visual layout. Your headings, page breaks, tables, and signature lines stay where you put them. That is why job applications, contracts, school submissions, and invoices are usually safer as PDFs.
Use Word to PDF when the document is finished and ready to share. Keep the original Word file for edits, but send the PDF when the recipient should read, print, or archive it without changing the layout.