It is strictly not necessary to change the prefix of each photo, as I follow a well-defined method of arranging the photos as per decade > year > month as well as decade > year > month and event.
But there is that random chance that I may want to create an album over multiple years. For this purpose, I will need to put many photos into one directory. When viewed as per their file names (which is the sort order I use commonly), all photos will appear mixed. Unless the file view is arranged by date, the photos will not appear in the proper sequence. By using a well-defined prefix with each file, it will ensure that photos appear in the same sequence when displayed by filename or by timestamp.
To rename files, I use a Windows batch script
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That is correct. It is not necessary to rename the files if they are copied into proper directories. But, as the number goes from IMG_0001 to IMG_9999, there is a remote chance of the file names clashing, if you decide to copy selected images into one single directory. For example, you may want to create one directory of favourite family photos over the decade. In such a case, there is a remote chance of a name clash.