This is going to sound dumb, but here goes. Our new house has mains water piped in.  It seems clean to view, but we haven't tested it yet.  Question, what is the purpose of the  water tank that every house seems to have,  why do some have two, how do we know that the mains water is filtered and if not hod do we arrange that and how do we get hot water and do we even need it?  As I say, dumb, but my wife  is not able to explain any of this to me in English that I understand, just saying that we need a tank. Not very helpful when you want to understand the why.    

I have read the various posts, just in case someone  comments, but I still don't see the rational and the various options presented . 

The reason is that when the mains water goes off, which it will from time to time for various reasons, your tank will tide you over.
Simple as that.

You may have also noticed that you have a pump somewhere near the tank.  That will come on when you turn on a faucet and the mains pressure is low or non-existent.  When the pump comes on, it is drawing water from the tank.  The tank re-fills "automatically" when the mains pressure is adequate and will not overfill due to a ball-cock shutoff.

 

You will get hot water from electric heater units although they are common only for showers.  Solar heaters are getting popular and guessing they will become standard for new build at some point.

Mains can not work for weeks at a time (whether that is due to piping being fixed, the village maybe high so not much pressure sometimes, whatever). 
 

No tanks will then mean no water. 

1 hour ago, wildewillie89 said:

Mains can not work for weeks at a time (whether that is due to piping being fixed, the village maybe high so not much pressure sometimes, whatever). 
 

No tanks will then mean no water. 

& that is why people have several - I would be looking for at LEAST 2000 ltrs in your tanks

43 minutes ago, BEVUP said:

& that is why people have several - I would be looking for at LEAST 2000 ltrs in your tanks

Yep, I personally have 2x2000 litre tanks. Water has been off for a week (due to maintenance), and barely used anything. 1x2000 litre tank would be sufficient for us (wife, baby and I) - as garden is watered with dam....but better to be safe than sorry. 

Many thanks for the replies.  It is indeed simple, so thanks again.