Explain the purpose of using multiple worksheet within one spreadsheet file?

There are many reasons why you would do it. For example you could have a business with different offices in different regions. The head office might put figures from the regional offices onto different sheets, like their amounts of sales, and then have one sheet which totals them all up. They could then see the individual offices and the totals all in one sheet, rather than having them in separate files. That is more efficient and saves space and allows you to do lots of calculations from figures on the different sheets. It is also better than have the different regional figures all together in one single sheet, as it would make it harder to control and navigate through. They can lay things out in the exact same cells in each sheet, making calculations easier to do. So the total sales in each sheet could be in cell D54 and knowing that they are all in D54 on each sheet can make it easier to do calculations to total them on another sheet.

Another reason might be to have things that are loosely related to each other all in different sheets in one spreadsheet. Using a business example again, they could have sales in one sheet, purchases in another, wages in another, stock levels in another and so on. All of these are aspects of their business. There would be some connections between some parts, so they could have calculations that might involve their sales and their stock levels. It can be handier to have them together in several sheets, rather than in separate files.

So there are many advantages to using several worksheets within one workbook.