What IIF is
IIF is QuickBooks Desktop's import and export format: tab-separated, with several tables interleaved in one file. A line starting with an exclamation mark defines the columns for a record type, and the data lines that follow begin with that type's keyword.
QuickBooks Desktop users, and the bookkeepers who have to move data into and out of it.
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 IIF to Excel
The largest table in the file is used, which is almost always the transactions. A typical export holds TRNS rows for the transactions and SPL rows for their split lines; those are two tables, not one, and combining them would produce a column layout that fits neither.
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.
The largest table in the file becomes the spreadsheet. An IIF holds several tables in one file, each introduced by a line starting with an exclamation mark, and the transactions are almost always the biggest.
The keyword column that begins every data line is dropped. It repeats the record type on every row and carries nothing you did not already know from the header.
Split lines are a separate table and are not merged into the transactions. A transaction and its splits have different columns and different meanings, and stacking them produces a sheet where half the rows are the wrong shape.
What carries over from IIF to Excel
IIF records 2 things about a table that Excel has no way to hold.
Excel wants a type for each column, and IIF does not record one, so each column is typed from what its values look like. A column of digits that should stay text - a zip code, a phone number, a leading-zero id - is the usual thing to check afterwards.
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.
IIF can be read one record at a time. Excel has to be complete before it means anything, so the whole table is held in memory while it is written and a file that gets cut off part way through is not partly usable - it is unusable.