XLSX is awkward for scripts, databases and import tools that expect plain text. JSON is what APIs and most programming languages read natively. XLSX is at home in spreadsheets with formulas, formatting and several sheets; JSON is the usual choice for APIs and data exchanged between programs.
The first row is read as column names, and every row after it becomes one object, 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.