Caixa d'agua?

Today there is a water "outage" in 350 neighborhoods in Belo Horizonte.  COPASA (the water company) is supposedly doing some work on a water plant to help increase with production.  The interruption in service was supposed to begin at midnight last night, and the water will be back before dawn tomorrow morning.  I just happened to see this because a Brazilian friend posted a notice on Facebook.  I wasn't sure at first about it (you know, some of those crazy Facebook/Twitter "news" reports).  But once I confirmed, I felt it my duty to spread the word.  I felt a little alarmed that so little was being done to notify the 2 million residents that they would be without water for more than 24 hours.  NO WATER!  I posted it on Facebook, and told our sindica (building superintendent).  Again, I was surprised by her blasé response.  She said she might post a notice in the building, but when I saw her later she said she didn't think that it would really affect us, because we only have 20 people in our building, and the caixa d'agua (water tank) should last the whole day.  

And this got me thinking about the caixa d'agua.  Practically every apartment building and house in Belo Horizonte has a caixa d'agua.  Why is this?  At first, I thought that these large water containers were the actual source of water for the residents, that is, a truck delivered water to the caixa d'agua.  But how the heck does the water get from the truck to the top of the apartment?  This is the faulty logic (stupidity) of the fresh-off-the-boat expat.  But after 3 years of living here, I still haven't figured out why they exist in such abundance here.  A brief google search didn't help much.  But here are my guesses:

1.  There used to be more frequent water outages.  The caixa d'agua is a remnant from these days when the water would randomly go out, and was insurance that there would always be a supply of water.

2.  Gravity.  Perhaps there isn't enough water pressure coming from water from the street, so a roof top container will provide the gravity needed to pump that water through the pipes.  

3.  Dengue.  Brazil really doesn't want to eradicate Dengue, and is committed to mosquito breeding.  

Ok, that last theory is a little crazy.  But most of what I found online were yahoo answers to questions like, "Why do I have to keep my caixa d'agua covered?"  and "why is it important to clean my caixa d'agua?"  Ewww.  

That being said, I have experienced the cleaning of the caixa two times (last crappy apartment, and current nice apartment).  We were without water for about an hour.  That was more hardship than was experienced today.  As far as I could tell, there was water all day, everywhere I went!  The pool at the club!  Nice long showers for all in our apartment today!  Our neighbors hosing down the sidewalks in front of their apartments!  The neighbor downstairs that ignores the rules and washes his car in the garage!

Anyone out there care to educate me about the caixa d'agua?