Many many people know Excel and use Excel. So many OR projects start with some Excel spreadsheet. And that is why being able to read from and write to Excel is key. You may even start the project with the Excel solver.
Moreover Excel is a common tool when companies choose plugin optimization instead of packages, custom or tailored optimization.
Excel remains extensively used in industry for non-OR applications. That means that if you are doing an OR application that does not require access to a database, there's a good chance the data for the application will come to you in either an XLSX or CSV file. On the flip side, when it comes time to convey the solution provided by your application, it is ...
I managed to understand why the pace of building up the model using Open Solver slows down after a while.
The reason is because there were several (in)equations without values, which was why model building was initially fast.