When should I use/not use '-w' for fastboot flashing?

When should I use/not use '-w' for fastboot flashing? Say I were to be manually flashing each img in the zip of a factory image . Why one way or another? Do certain .img's need to be erased first? I recently had a misflash where my internal storage was reduced from 64gb to 32gb and I am wondering if it could have been avoided. I resolved my particular problem by using Format Data in TWRP.