Why the need to "top off" a rez?

So I was wondering; I've seen a lot of people say that it is good to top off a res each day with roughly a gallon of half-strength of the in-res nutes (dependent upon res size of course).

If the res is big enough, say your flood table only holds maybe 10 gallons (24*24*4) and you load your res with 20 gallons of water, as long as algae isn't a problem, couldn't you just leave it without topping it off until water change?

Depending on when you change your going to lose water everyday so toppingkeeps it at the correct level. You don't have to top with jute water just top with plain water until you have replaced the total amount of the res then replace.

If you aren't using a ppm meter you must monitor water level as the alternative way to keep the right nute level. If you monitor ppm the water level doesn't matter so much, just having the correct ppm level. That is unless it goes dry which happened to me with my first ever grow in an aerogarden with a one gallon res. Anyway, when my roots get well into the water I often let the water level fall on purpose to make the roots reach for deeper water. Just remember that ppm can rise quickly if plants drink a lot of water. At that point you need to add water to bring the ppm down, but not just to get the water level to a certain place.

This video does a nice job of explaining the need to top off the res.
[youtube]i6eeMJp7ZJs[/youtube]