What JSON is
JSON is a text format for nested data: objects with named keys, arrays, strings, numbers, booleans and null. It has no date type and no comments.
Every web API, most configuration files and anything that talks to JavaScript.
What XLSX is
An Excel workbook is a zip archive of XML parts describing sheets, cells, formulas, styles and charts. The .xlsx extension is that format; .xls is the older binary one from before 2007.
Finance, operations and anyone who received a file from finance or operations.
What changes when you convert JSON to XLSX
Four shapes all read correctly: an array of objects, an array of arrays with a header row, an object of column arrays, and an object keyed by row id. A single wrapper key holding the array, which is how most APIs reply, is unwrapped automatically. Nested objects are flattened one level into dotted keys; anything deeper is kept as JSON text in the cell, because a table cannot hold a tree.
You get a single sheet with a header row and column widths sized to the content. Values are written as text unless type inference is on, which keeps identifiers like 0044 or 007 intact.
What carries over from JSON to XLSX
One property of the JSON has no home in Excel, and it is worth knowing which before you convert.
JSON can hold a value that is itself a list or an object. Excel has only flat cells, so nested values are flattened into one cell rather than being spread across columns.
The keys stop being repeated: Excel writes the column names once at the top, so the same table takes noticeably less space.
Both formats record types, so numbers, booleans and nulls survive as themselves rather than as text.