Perceptions and memories are often biased, inaccurate, or distorted over time. An individual can only experience a tiny fraction of all possible situations - knowledge gained is limited. Public in the sense that methods and results obtained by one are shared with others. Presents a consistent, stable reflection of the underlying construct. Standardization - everyone who measures the construct uses the same instrument in the same way.
Standardization improves objectivity because everyone uses the same procedures.
It also improves communication and comparison. You can compare data to other organizations, other time periods, etc. Otherwise we have to argue representativeness of the subjects Linking Science and Practice - Academic Journals and other scientific reports.