To get a good understanding of Tenkara lines and their functions, you need to develop a good understanding of fixed line rods and why they are designed in the ways that they are. This is kind of like asking the question: Which came first - the chicken or the egg? I do not know for sure but I believe the fishing line predates the fishing rod. But who ever figured out that a flexible lever could be used to cast a line and fly put us on the very productive road to where we are today.
In the interests of getting a handle on understanding T-rods and their casting abilities, please read the following articles and all the comments following the first article. Both of the articles will help explain why some T - rods will cast light level lines much better than heavier tapered furled or woven level lines. They will also explain rod tip heaviness, balance, and why we need to put lines on our rods that weigh enough to load the rod and bring out its action for easy, accurate casting.