One user at a time : In the instance where there are a number of people required to work on a spreadsheet, they can only access it when no-one else is working on it.
Protection of formulae : How often have you forgotten to protect the cell contains the formula that took you hours to test and perfect. It takes one instance of honest human error, and that formula is gone.
Over time, the spreadsheet grew into what we see today, in the form of Microsoft Excel and a number of similar products on the market. The power of the spreadsheet has grown almost exponentially, allowing the user to create customised formulae, charts, pivots and so on. Worksheets can be linked and updated automatically.
Selling spreadsheets helps you to build a catalog of spreadsheets that you can take with you and use in various areas of your life. Spreadsheets built for sale are usually built for flexibility and would be customizable to your life, or job, or new life, or new job.