Export to Excel is a feature in Excel which is available in Power BI for a very long time. It allows report users to export the data from a specific visual in the report to an editable Excel file. After exporting, they can do whatever they want. For example, sending the data to others via mail, transforming or manipulating the data, start building new reports based on the Excel file and many other things. The export option can be used by clicking the ellipsis on the right top of a visual (if the visual header is enabled).
If you have all export functionalities enabled, users can both export underlying data and summarized data. The difference is mainly raw data or only data as visible in the chart where you clicked the export button.
Read on to understand why this might not be an unalloyed good.