The piston really needs to at TDC or close to it for a conclusive leak down test. Otherwise the valves may be open slightly, rendering the test invalid. Some engines may pass at BDC, others may have a bit of overlap and fail.
The results are more conclusive than a compression test if trying to troubleshoot a misfire. A leak down test not only shows the cylinder's ability to hold pressure, but where it's going if there is a leak, therefore narrowing the problem to a specific component.
A compression test is usually done to evaluate engine condition and to pinpoint a weak cylinder.
Having both tools is best, but you can do more with only a leak down tester than you can with only a compression tester.