Reusability
Reusability means reusing code written earlier, may be from some earlier project or from the library. Reusing old code not only saves development time, but also saves the testing and debugging time. It is better to use existing code, which has been time-tested rather than reinvent it. Furthermore, writing new code may introduce bugs in the program. Code, written and tested previously, may relieve a programmer of the nitty-gritty. Details about the hardware, user-interface, files and so on. It leaves the programmer more time to concentrate on the overall logistics of the program.