There's no reason you have to add new podcasts to a podcast feed. But many podcasts are "shows", and once someone subscribes to your podcast, the idea is to add more material on a regular basis.
For example:
1) You could add a new podcast to an existing feed:
2) Then, if a person is subscribed to the podcast,
they can click the Update button in iTunes: 
3) And then iTunes will show that a new podcast is available and they can "get" it if they like.
