[scott_hardy]

These reasons are (casually) mentioned in the post:

* "the deeper your reservoir the more efficient the water gets pumped. As the water gets consumed, the pump gives less output"

* "If the reservoir runs dry the motor won’t catch fire. That apparently is a thing water pumps."

* "Since the water is only going through a simple tube and not an expensive motor, you can pump a nutrient solution. If you want to pamper your plants, we don’t."

[JshWright]

An impeller driven pump moves a fixed amount of water per revolution.

There are very few pumps on the market nowadays (even the super cheap ones) that don't have run-dry protection. In any case, no pump is going to catch fire running for 20 seconds.

It is very common to add fertilizer to planted aquariums. It's dissolved in the water, and therefore makes no difference to the pump (it doesn't change the "pumping" properties of the water at all).