People take transit because it takes them to where they want to go and they value their time. That means that, as long as there is room for them, they tend to take the route with the shortest travel time. Downtown circulators generally perform very poorly, because once you account for the waiting time, transfer time, etc., since they only travel a short distance (and they likely still have to walk from their final station) they don't end up saving that much time. Generally, people don't like travelling in circles.
Lines that go a longer distance in a direction are more useful because they accumulate more time savings. They also allow for more order/destination pairs to
be served through a grid system.
Transit that is fast (large travel times savings) is what we want, because it is in a non-overlapping and therefore complimentary role with walking and biking. Slow or short transit (e.g. ~2 km) just ends up cannibalizing trips from walking or biking, since those short trips aren't ideal for driving to begin with.
This shows up in ridership studies. For instance, Montreal was looking at a downtown circulator line. They created a bus route to mimic the route, and it performed very poorly. Sydney created a downtown circulator,
and they eventually decommissioned it because ridership was so poor.
The goal of a new subway isn't just to "service" an area with a station, it is to expedite travel in a way that is useful for people.