I record in the field with a portable Zoom recorder, so I don't need an audio interface to plug any mics into. I'm just wondering if when I'm editing a podcast with lots of scenes and sound, if I need an external sound card to do the best quality sound design, or if it makes no difference.
Well, you need to be able to hear the podcast in order to edit it. Often the zoom recorder and a pair of decent headphones will be a good solution.
What you might want to do is probably to remove really low frequencys, say below 100 Hertz for spoken word. Additionally make sure that the relative volumes are about the same, getting blasted with high volume music followed by very low volume spoken word will not be loved by your listers.
The general idea however is that a better listening equipment and environment allows you to hear more exactly. As you can hear more exactly, you can better use tools in order to correct errors in the sound and otherwise make it more pleasant for the listener. But you need to have the tools and experience to use them correctly in order to benefit from the better listening.
We may compare to when songs are created professionelly. The last stage before sending the song to the public, say on a CD or whatever, is known as mastering. There are professionells in the world that work full-time doing mastering, using extremely expensive speakers in specially built rooms. The do charge quite a bit to do a mastering. This would be totally over the top for a typical pod cast.