It loads external boot loader/firmware update tool file (usbloader.bin) via emergency serial port available if the firmware is corrupted or boot pin (test point) is shorted to the ground.
Original Huawei USB loaders erase NAND flash when loaded. usblsafe.bin "safe" loaders are patched to disable flash erasure procedure.
You should never need to erase flash in normal circumstances as it would remove all custom factory data (IMEI, S/N, radio calibration).
This repository contains "loader-patch" automatic patcher to convert usbloader.bin to usblsafe.bin.
Moreover, balong-usbload would patch "unsafe" usb loaders automatically, and if it failed to do so, won't allow you to load unpatched loaders without `-c` flag to prevent flash erasure.