"Car key" and "concert ticket" are examples of compound nouns. Seperately, the words make sense, but when you put them together, they imply a brand new meaning that can't be made with only the words by themselves. So the two words actually function together as one: one noun, one word. Alternatively, "today's podcast" is not a compound noun. The subject is the podcast, but that podcast is made more specific by adding the qualifier 'today'. The podcast belongs to today, it is possessive, so the " 's " is needed to dennote that. Does that help?

Because of the "s". I imagine that's obvious.