What if I have a friend on the other side of town and we are meeting up at a restaurant on their side of town? Or maybe there is a high speed rail connecting a few cities and now I can visit my parents the next city over by taking the train. Or maybe I didnt manage to find a job in the more walkable part of town (we cant fix cities over night) but the transit hub can connect me to my job. Or maybe I usually walk the 20 minutes but I injured my leg and its only 5 minutes of walking if I take the bus.
I think transit belongs within a well designed city and for intercity connections. Even with the best urban planning, some cities will just be too big to get everywhere in the city just by walking. Some people might be fine staying in their neighborhood but others will want to see other people, try different restaurants, shop different places.