Often times you need to be sure that you have valid data in your spreadsheets as input to avoid embarrassment or even disastrous consequences. Especially if it relates to investment, accounting, sales.
For example, you need to be sure that user inputs only valid dates, valid amount, non-duplicate entries or data from a pre-selected list and not any value that the user fancy etc. in a cell.
Google sheets have simple ways of doing this and that saves a lot of effort from your side and gives confidence that the data in the spreadsheet can be trusted.