The specific advantage of documents with multiple worksheets is that you can also perform calculations across multiple worksheets.
To do so, specify the name of the desired worksheet before the cell address (separated by an exclamation point). For example, "Sheet1!A1" refers to cell A1 on worksheet "Sheet1".
An example
Let's assume that you want to create a profit and loss statement. For this purpose, you have created three worksheets named "Sales", "Costs" and "Profit".
You now want the value in cell D2 of the "Costs" worksheet to be deducted from the value in cell A8 of the "Sales" worksheet and to appear in cell C9 of the "Profit" worksheet.
To do so, switch to the "Profit" worksheet. Navigate to cell C9 in this worksheet and enter the following formula:
=Sales!A8-Costs!D2
After pressing the Enter key↵, cell C9 in the "Profit" worksheet will contain the result of your calculation.