In a sense, the purpose of the Transport layer is to compensate for the fact that the Link and Internetworking layers might lose data. When the two lower layers lose or reroute packets, the Transport layer works to reassemble and/or retransmit that data.
The existence of the Transport layer makes it possible for the two lower layers to ignore retransmission and rate-limiting issues.
Part of the goal of a layered architecture is to break an overly complex problem into smaller subproblems. Each layer focuses on solving part of the overall problem and assumes that the other layers solve the problems they are supposed to solve.