Number of reasons, firstly if we are dehydrated our bodies will try to hold on to all it's current fluids so we urinate and sweat less, it's also a great appetite suppressant, and often when we think we're hungry, we're actually just thirsty. Our digestive systems need a good amount of water to digest food properly, and our fat stores are only metabolised in a fluid media so if we want to burn fat we need to be properly hydrated so you need to drink enough water so that fat can be flushed from your system. If we’re not getting enough water it’s difficult for our bodies to function properly, much less to flush out extra fat we want rid of.
We are more energetic when we are hydrated too so we move more and sit on our arses less