Seems I read somewhere online, or in a tutorial, or somewhere... that adding stops to an area will make an area more popular and more desirable. You build a "market" for some buildings. As a result more people will want to drive there as well.
That seemed to be the theory. And its probably set up that way in the game, the problem is that we see it from a birdseye view and relationship seems to "contrived".
It should probably be more subtle.
They need to find ways to keep the cities dynamic and changing. And this is one way to do it. But maybe it needs to be toned down a little.
If traffic patterns stayed constant and never-changing, the game would be too predictable. Having the traffic react to your actions adds a really nice dynamic to the game. But yeah, maybe they need to tune it so its not so obvious that placing a line generates so much traffic right on that line.
For comparison, there are obviously other things that fluctuate and change. If they didn't, then things would be too predictable and it would be too easy to always find the "optimal" combination of things. The dynamic-ism is what keeps the game interesting. For example, the economy changes up and down. This affects how much you pay workers and how much you charge ticket prices. When the economy is doing well, you have to pay your workers more... if you don't, then your satisfaction will go down. Raising your wages instantly makes satisfaction go up. So you can control what the game has dynamically thrown at you. Failure to do so will have effects.
Also, the city grows. Buildings spring up in different areas and create new places to go to. And if you don't figure out who wants to go there or leave there... and build transportation systems to take care of that... then people will start driving instead and clog up new roads.
These factors make the game actually enjoyable. If it were not for these factors, then you'd probably only play through the cities one time and then get bored.