Mosaic has found many very useful applications of testable requirements sizing measures.  A few are introduced below:

Estimating System Size -- Estimating system size is beneficial to the success of projects by communicating the system size to management, developers, testing staff and end users.

Tracking the Change In System Size Over Time -- Managing system scope is critical to the success of many projects.  System changes usually affect system size, but the impact is often ignored and not properly managed if the size of the system is not measured and tracked.

Quantifying Understanding of Requirements -- The number of intermediate level requirements (ILRs) associated with each high level requirement (HLR) may indicate that not enough is known about the HLR, or that the system is being designed or developed beyond expectations.

Measuring Test Case Coverage -- Testable requirements can be used to measure the thoroughness of test cases.  For each high level requirement, the percentage of coverage may be calculated by dividing the number of testable requirements covered by test cases by the total testable requirements.

Calculating Earned Value -- Earned value is a measure of completed work expressed in terms of the budget assigned to that work.  It represents how much of a system has actually been developed, as opposed to how much time or how many resources have been expended.  The testable requirements measure has been suggested as a good earned value measure that is applicable across the system development life cycle.

Assessing Risk -- A testable requirements size estimate can help identify the overall project risk based on size, and also help to evaluate the reasonableness of the project estimates and highlight the higher risk areas.