Jonathan, it's just an option to work within Tableau when all data are available in Tableau.
It is also an option using Tableau to generate Case Statement for SQL or other languages. It only needs copy and paste those Cases into a blank worksheet, plus a calc field.
Anyway, use whatever is handy. Tableau is just one of the options. Google Sheet is another, besides Excel.
I think the excel file is every useful for my work. However, I'm not familiar with "IN" List. it's seem useful but I don't know how can I use it in Tableau?
If it's possible , could you give me some example that you use "IN".