Advantages of creating measures:
They can offer greater relevance and validity than using existing measures.
Advantages of using existing measures:
Creating good measures takes time and energy, both of which could be saved by adopting an existing technique.
Of greater scientific significance, measures that have been used frequently by other researchers carry a body of possible comparisons that might be important to the current evaluation.
Finally, measures with a long history of use usually have known degrees of validity and reliability, but newly created measures will require pretesting or will be used with considerable uncertainty.