It does prebinding.
Binding is the process of determining memory addresses to call for functions in libraries. Normally, this depends on the memory location at which the library is loaded, which can vary. Therefore binding must often be done whenever an application and the libraries that it uses are loaded into memory.
However, prebinding is calculating these addresses for predefined memory locations for libraries. This way, the binding doesn't have to be done when an application loads. Prebinding must be done over whenever new applications and libraries are installed. A different layout must be used if the system uses different libraries. Therefore it's often done at the end of installation.
So it optimizes application startup. You can manually do this whenever you want:
sudo update_prebinding -root /