In any case, appending implies that a second (third, fourth....) entitity is appended (surprise!) to a first with the result (in the case of files) being the first file extended by the content of the second. IOW, the original first file is changed.

Concatenating, OTOH, implies building a new file (entity) by combining the originals... without modifying those originals, at all.

So why is this minor, pedantic, but non-trivial? Because precision of language and thought are critical assets for a programmer.