Lets say on Day 1 you dos pure ammonia. By Day 3, all the ammonia has been converted to Nitrite, but not yet to Nitrate. At this point, the Ammonia eating bacteria have no food and will go dormant. Then you dose again, and it takes one day to convert it all. That means that the ammonia eating bacteria population increased, but then has to go dormant again.
Now, if you add a shrimp, it decomposes at a fairly steady rate, releasing a constant stream of ammonia into the water. You'll know the ammonia eating bacteria population is good when it converts faster than the decaying shrimp releases it i.e., you know the shrimp is rotting, but can detect no ammonia. You're cutting out the dormant period between dosings.
So while you can dose, then leave for three days, the cycle may be faster if there is a constant source.
Does that help?