A boiler typically is a closed loop and runs at a higher temperature than a WH. Codes typically restrict the max temp of a WH for domestic use, and safety means you rarely need more than say 120-degrees. A boiler often runs at closer to 200-degrees.
There are a few common ways to heat domestic hot water...a stand-alone tank, either electric or gas, a demand system that uses a heat exchanger in the boiler itself, or an indirect tank that is essentially a separate zone off of the space heating boiler. The fuel used could vary from electric, natural gas, propane, to fuel oil.
My guess is they have separate boilers and tanks so that the utilities can be billed separately...this keeps someone who wants to economize from subsidizing somebody that likes it tropical (although they get some parasitic heat from their neighbors).
So, not sure if this answered your question.