The usefulness of exporting your portfolio, and re-importing it may not seem obvious at first, but it is a very safe backup mechanism, and can also be used if you move to a different device.
When tap Export Portfolio, you will be shown a share dialog by your system (this may look different between Android and iOS). You can choose to email the backup to yourself, or save it to your local device or the cloud. The file format is proprietary and cannot be read by any other application.
The files you exported can be imported using the option Import Portfolio. The app will ask you to confirm that you want to overwrite your current portfolio with the import first. If in doubt, export your current portfolio to a different location if you want to keep it.
Export as CSV might be a useful option if you want to analyse your portfolio, or parts of it, in Excel or Google Sheets. The app will ask you if you want to export the whole portfolio, or just the current visible filtered strategies.
Users with Premium subscription have to worry less about backing up their portfolio, it gets saved in the cloud automatically, and synchronized between devices using the same login.