Microsoft Excel is the most widely used spreadsheet application in the world, which makes XLS/XLSX an ideal reporting format for .NET applications. Some of the advantages of using Excel reports over other alternatives [Like HTML, PDF etc.] are:

• The report generated can contain rich formatting like Charts, Pictures, multiple worksheets, formulas etc.
• The end user can use the Data Visualization power of MS Excel to manipulate the generated reports. For Example, the end user can generate several charts to analyze and understand the sales numbers in a product sales report.
• The popularity of MS Excel guarantees that the end user is already familiar with using MS Excel.