The only reason would be for using the external with a console
Be patient with me: Why do you want to format a 3 TB drive to fat32? Have you initialized it as an MBR drive or a GUID drive?
Unless there is a very specific need for it, you should probably use some other format (or get a smaller drive and use this one for your data on your PC).
Of course, I don't know what you need this for.
, since PS3 only recognize HDD/Externals in the FAT32 format. So if you are stuck with a lower storage console or one that came with 4GB SSD and no HDD you might need to format an external to FAT32 and use it as your console's HDD.
The other option is to buy a 500-1TB 2.5" laptop HDD and do the internal HDD upgrade yourself.