To add information into a FITS file header, it is cumbersome to load it, modify the contents in memory, then dump the results as it is currently done by fitsedit. One way to avoid that is to modify the file in place, but that assumes that you will use some existing space in the file to insert your data.
replacekey is meant to replace some cards in a FITS header by a card you provide on the command-line. It acts in place, making it extremely fast. The ideal is to use a software that will put placeholders in the output files it creates. You can then insert information in place of the placeholders, by declaring to replacekey what the placeholder name is, and what you want to replace it with.