The idea was a little bit different.
If you copy all the content from an USB device to a PC HDD and copy them back without formatting (USB device),
file system will (mostly) overwrite previous data, but the files still might be fragmented.
Or maybe in other words it will not start to create data from the first sector, but from the last that was used.
Because deleting\moving files are not fully erasing them from any device.
If you format your USB device (even fast, which is is a way faster), files will be write from the 1st sector
and they will be continuous (not fragmented)...
In a simplified way. My English is quite bad, so many people will have a problem with understanding what I just wrote.
That is why I previously wrote these 3 simple steps.
Maybe it is safe to format. But I wouldn't be sure it is different than just deleting (deleting all the contents obviously). Formatting simply tells that i.e. from sector 0 to sector 100 there's no data. You get the same result when you delete everything.
At least in my experience, data is always installed defragmented after you delete everything in the device.