It has to do with gravity and the pressure of gravity against the pump and it's ability to go against the pressure. Typically with aquariums you have a 4-5 foot distance from your sump to the top of the tank and a pump like the Mag 5
that can produce 500 GPH
without any pressure will only produce 250 GPH
with 5 feet of pressure against it.
If you were trying to power a waterfall in an outdoor pond that was 8 feet up you can see you would need a pretty powerful pump to get the water up or else you would have very little flow.
Also each 90 degree bend in plumbing (using PVC
for example) counts roughly as 1 head pressure. Each 10 feet of horizontal piping reduces 1 head also.
So to sum up if you have a tank with a 600 GPH
overflow and you have 4 90 degree bends and your sump is 4 feet below the tank to get close to 600 GPH
flowing through your tank you would need a pump that is rated at 600 GPH
with a 8 foot head (like the Mag 9.5
It's better to buy a slightly stronger pump and slightly restrict the flow using ball valves (if it can handle it) or T off the return to have some go back into the sump or a fuge.