Values are indicators of a particular type of aptitude the value of objectivity shows the aptitude of the rational decision-maker. And civil services require a particular type of aptitude which can be tested through some values.
A bureaucrat has become directly accountable to citizen-customer. He has to respond to the moral universe of the citizens. Thus, foundational values work as a guiding light.
He has discretionary powers; therefore, he must be provided with guiding principles to prevent abuse of power.