Google is chasing the "ease of use aspect". A mobile OS has to have a notes app, a mail app, a chat app (in Google's case abundance of chat apps) and a podcast app. Google can track your personal podcast preferences, listening habits, etc.
Personally, I would not choose to use any default Google app. There are lots of better alternatives.
I guess most that will use YouTube music will see it as a convenience type thing. If I was in say android auto and my default search is on YouTube music being able to have it just play the podcast I was looking for would be very convenient.