Flashing an image vs copying files

I would like to understand how is flashing an image on device or partition is different from copying same set of files to the device?

This is different because being able to copying files does require:

• a partition to be defined on the drive/eMMC/USB key/SD card,
• a file system to have been created/formatted in one partition.

When you flash an image, you flash all of the partitions, file systems, files in the file system that were existing on the drive/eMMC/USB key/SD card the image was created from.

Bottom line, you cannot copy files on a blank drive/eMMC/USB key/SD card, but you can write an image into it.