There is an old management adage “We can't manage what we don't measure.” Unless we measure something, we don't know whether things are getting better or worse. With a clearly established measurement metric, we can quantify progress and adjust our process to produce the desired outcome. Software development costs can be controlled by managing the unit price of the software project, and measuring the number of units delivered by the software project.
Function-points measure the size of the functional user requirements. It is being widely accepted and has been used in the software industry for almost 30 years now.