Why change the default location
The default location of a profile folder is usually on the C drive. There is times that it is beneficial to relocate the folder to a different drive.
SSD drives are more expensive than regular drives but they are faster. Because of this, some use a smaller size SSD drives. They store the operating system on the SSD drive and store files that they create on a regular drive. To do this, they have move the default location to the other drive.
Some people put the operating system on one drive and the files on a separate drive because they share the files with two or more computers. They store the information at a specific location and share that location over the network to other computers. This way they have the same files on more than one computer.