Would someone please explain whether there is any difference, and if so the pros and cons, of storing functions/subs used across multiple workbooks in the Personal.xls file vs creating an XLA addin?

I know an XLA can be linked in the references so that functions/subs can be called from other workbooks as if they were a part of those workbooks. I'm not sure that can be done using the personal.xls file without using Application.run. If, however, the personal.xls file can be lined in the references then I fail to see the difference between the two options.