When is it important for scientists to describe objects completely?
It is always important for the scientists to describe the properties of an object completely. Scientists work by observing phenomena or objects and make certain hypothesis to describe the observations. These hypothesis should be testable and verifiable. Scientists then design test/s to verify their hypothesis and use the information to modify the hypothesis, reject it or accept it. This process of verification continues, with each successive success increasing our confidence and each failure causing its modification.
In order to successfully carry out the scientific process, one has to completely understand the object, including all its properties. For example, to determine the degree of thermal expansion of a ball, not only do we need to know its dimensions, but also its melting point (among other parameters). An incomplete knowledge will leave the tests and process open to possible failure, something we can avoid by meticulous understanding of the object itself.
Hope this helps.