We also know that the acceleration due to gravity is 9.8 m/s2.

A standardized value for acceleration due to gravity was therefore needed.

Usually the goal is to measure the mass of the sample rather than its force due to gravity at that particular location.

Since acceleration due to gravity is the same, why do objects fall at different rates under water?

What you are observing is the difference between air and water, not differences due to gravity.

A familiar example is potential energy due to gravity.

Any dramatic acceleration, whether due to gravity or some other force, would have such a result.

As a result, each experiences a smaller acceleration due to gravity.

A falling body is accelerated at 9.8 m/sec2 due to gravity.

An official value for acceleration due to gravity was needed.