The last version to store code in a sheet was xl95 and those were macro
sheets--not worksheets.

And code is stored in module (a general module, a class module (like a userform
or behind a worksheet or behind ThisWorkbook)).  But all of it is in a module.

But those modules don't exist on their own.  They exist in the projects of
workbooks.  And personal.xls is a workbook that (usually) has code in the
modules that are contained in its project.

So you're going to put all your code in modules (in some workbook).

Lots of people have their own custom functions/routines that they want available
each time they start excel.  A common spot for these functions/routines is in
the personal.xls workbook.

But there can be other functions/routines that are only useful in a particular
workbook.  These routines could be stored in the same workbook that contains the
data.

And there are other functions/routines that are useful for a type of workbook.
Say every month you create a report for your boss.  You create the workbook from
scratch each time.  But there are routines that are particularly useful when
you're doing these reports.

But they're not very useful for the other 99% that you spend in excel.  These
could be stored in another workbook (just like personal.xls, but named something
else) that you open only when you need it.

In fact, sometimes people save these types of workbooks as addins.  They'll
build toolbars that are used to invoke the macros in the addin.  The workbook is
even more hidden than personal.xls is.  Most people don't need to get to the
worksheets (manually)--even if the code uses the data on its own worksheets for
some purpose.