XLSX is awkward for scripts, databases and import tools that expect plain text. XML is what many feeds, enterprise systems and older integrations require. XLSX is at home in spreadsheets with formulas, formatting and several sheets; XML is the usual choice for document-like data, feeds and enterprise systems.
The first row is read as column names, and every row after it becomes one <row> element, keyed by those names. A header such as address.city is turned back into a nested address with a city. A workbook with a single sheet becomes a plain list; one with several sheets becomes an object keyed by sheet name.