Not being able to delete files through the app is not a big deal, really. They're eventually going to get overwritten automatically in a few days anyway. Manually trying to manage SD card space is a huge waste of time and randomly deleting files can cause problems like
@kamkar talked about.
Only 50 video files can be locked before they start getting automatically overwritten. If they locked all motion detection files and your cam see cars constantly going by in the parking lot/garage, or people walking by, or you parked in front of a tree on a windy day, your locked video of someone doing something to your car will get overwritten pretty quickly.
With a 128 GB SD card, you have 7 to 17 hours to retrieve any video file before it's overwritten, depending on image quality setting. Double that with a 256 GB SD card. If your cam has internet access it'll send you a push notification with any parking event, motion or impact, if you configure it that way. You get immediately notified that something happened and you can transfer that video of interest directly to cloud storage from your phone without even returning to your car.