What is the Concept of Marginal Cost of Capital?

The marginal cost of capital is nothing but the weighted average cost of the last dollar of capital rose which is expected to be the cost of another unit of capital raised.
It has been seen that when the company continues to raise capital then at some point the marginal cost of capital can be higher than the weighted average cost of capital. It is true that firms will try to their best to remain attached to the optimum capital structure when they prefer to raise the capital. But firms face an increase in marginal cost of capital as capital level increases while sticking to the desired capital structure.
Marginal cost of capital has proved to be a yardstick in several decision making processes when it comes to the rise in the capital while sticking to the desired capital structure.
Also add a brief discussion about the WACC. That would, kind of, complete the whole discussion.