You want to maximize cardiac output (CO)....and this is measured as stroke volume (the amount of blood pumped in one beat = SV) times heart rate (HR)...that is, CO=SV x HR.
The more you train, the higher your SV becomes...so at rest, your heart rate drops because you pump more blood per heartbeat.
So a low resting heart rate is just a side effect of athletic training.
By the way, the formula CO=HR x SV has limitations....if your heart rate goes too high (say more than about 200) the heart does not have enough time to fill between beats, and the CO won't keep going up.
I could go on, but I hope you get the idea.
And just because you have a lower resting HR or higher HRmax than someone else doesnt mean you are necessarily in better shape anyway.
P.S. The oxygen your body can deliver is the most meaningful number for athletic performance, and that depends on cardiac output but also red blood volume...that's why unscrupulous racers use epo etc to get more red blood cells.