iOS's Crash Reporting service stores its logs on the device errors occurred on. For private devices, this can be quite helpful for debugging customer app issues.
You can find these logs by going to Settings -> Privacy, then scroll down and tap Analytics & Improvements-> Analytics Data. You'll find a list of crash reports; Look for the app package name and relevant date.
Once you've opened a report, you can copy it to the clipboard or email it to yourself.
For more information, see Apple's documentation.