So any difference at all means that Einstein s theory is not strictly correct, and is at best an approximation to reality. If we can learn enough about the nature of any differences that happen to exist, they could point the way to a better theory. Most other theories of gravity predict some violation of Equivalence. The two leading theories of nature today, General Relativity and Quantum Electrodynamics, are logically incompatible with one another. So we know that at least one of them is wrong.
STEP is a cost-effective way of getting at least one more data point towards finding out which.
It s done in orbit. STEP s sensitivity in orbit comes from several factors. One is that the test objects fall around the Earth for a very long time without hitting anything, and their separation due to any Equivalence violation increases as the square of the time they fall. Another factor is that disturbances are much smaller in orbit. You can t measure an effect that s much smaller than the disturbances, and expect anyone to believe it.