When might I want to use APPXUTIL to check my file system?

If your file system (disk) fills up. Adding records to a file on a full disk is likely to corrupt the file.

After a system crash during which there were active users on a system. You should verify all files in the database before allowing users back into APPX. When certain records you "know" are in a data file, do not show up as expected on a report or input display, you might want to verify that file.

When a constrained child process fails to correctly retrieve data, you might want to verify the PCF of the child process.

For routine file maintenance. Once a month on your data files and on application design files that are being edited is recommended.