The Main Standard Libraries (MSL) are complete, configurable set of C and C++ standard libraries. So this library is necessary if using C or C++ to develop project.
To use the MSL library, you must also use a runtime library. A runtime library provides low-level functions that support high-level C and C++ language features, such as memory management and file system access.
For details, please refer to <Chapter 23 Libraries and Support Code> in Power Architecture Build Tools Reference, usually is:
C:\Program Files\Freescale\CW for MPC55xx and MPC56xx 2.10\Help\PDF\Power Architecture Build Tools Reference.pdf