The advantage of the dosing pump is I can discharge from my tank / sump regardless of gravity. During the spring, summer and fall I can run a long tube into my yard, or the tomato plants on the porch. During the winter when an outside tube would freeze I can route the output to a drain inside.
I also like to keep my tank as clear from tubing, pipes and wires as possible. This is one of the reasons I like to run a sump. My heater, dosing pump, etc is all in my sump which greatly reduces the number of wires and tubing in the tank. The sump sits at floor level (below the level of the drain for the kitchen sink) so I have much less of a water level drop so a much weaker siphon (none at all for the kitchen sink drain).
Using a dosing pump to pump water out of the tank would also work for a tank in the basement where gravity is your enemy.