What MySQL Output is
Not a file format at all. It is what the MySQL command line prints when you run a query: a box drawn with plus signs, dashes and pipes, with a line underneath saying how many rows were in the set.
Anyone who has run a query in a terminal and wants the answer in a spreadsheet.
What Excel 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 MySQL Output to Excel
The rules and the trailing row count are dropped and the pipes become column boundaries. MySQL prints an unset value as the four characters NULL, which arrives here as an empty cell, because that is what NULL means and because a literal string of NULL in a spreadsheet is worse than a blank. Paste it as CSV instead and every row becomes one column full of pipes, which is why this is its own format.
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 MySQL Output to Excel
One property of the MySQL has no home in Excel, and it is worth knowing which before you convert.
Excel has no separate idea of a header, so the column names become an ordinary first row and nothing in the file marks them as different from the data.
Both formats record types, so numbers, booleans and nulls survive as themselves rather than as text.