The basic logic behind this, plain text files bearing a different extension, is for the sake of easy grouping, or to set apart the original text files from the output files, especially when they are going to be stored in the folder.
Alternatively, a new folder is created to hold all the processed files, and the original files are kept where they were. Or a slight change to the filenames can be made, for instance, from *.txt to *_TAG.txt.
These are but different practices used by programmers. The more you get used to them, the less puzzled and the more understanding you will be.