For a very large chunk of the pokemon in Red/Green/Blue/Yellow I can generally think of some sort of purpose for their placement in the game. If I had to take a wild guess Farfetch'd exists to be taught Cut to. (you get them both roughly around the same area/time)
Now that I think about it thgouh, it would be kind of interesting if instead of needing a cut hm, you needed to have a Farfetch'd to clear through shrubs. And possibly apply it to other HMs as well. Could have been a more interesting way to encourage you to catch them all.