It's for when you want to have distributed data stores.

For example, if you're building out a doc to track your net worth, income, and spending habits:

- One main sheet that collates all the data required here (e.g. credit card statements, in-flows and out-flows from bank accounts, current debts and current assets)

- Have 3 more docs:

One that collates and summarizes your credit card charges

One that collates your bank statements

One that collates your assets and liabilities

///

Let's say my "credit card charges" sheet automatically pulls in my charges over API, and appends them to the "Charges" sheet in the "Credit Card.xls." Then the "Summary" sheet in this "Credit Card.xls" summarizes this information into something useful. My "Main.xls" (that collates all of my data into something even more readable and useful) can then pull data from this "Credit Card.xls" sheet (through API or locally), and automatically keep itself updated.

I could stuff this all into one single doc, with numerous sheets, but I don't want to deal with the cognitive overhead of having to navigate through an enormous amount of sheets I rarely (if ever) need to touch (again).

Perhaps I don't even have access to the physical "Credit Card.xls" doc, because my personal assistant automatically appends to it, and keeps it uploaded somewhere.