You may reformat an SD card/USB flash drive to:
Gain more device when the SD card or USB drive is full.You may clear or wipe an SD card/USB flash drive to:
Both of the operations will wipe all your data on the device but they are usually done for a different purpose. In many cases, you should clear or reformat an SD card, USB flash drive, pen drive, or hard drive for certain reasons.
Differences between erasing and formatting a memory card/USB drive