Using hot water helps to "bloom" the cocoa powder. Blooming is a fancy way of saying "enhance" the chocolate flavour to make it even deeper, richer, and chocolaty tasting. It also helps to break down any cocoa powder clumps (but still sift it!). TIP: If you want to bring out the flavour, even more, try substituting the hot water with hot brewed coffee. The coffee flavour will not be there but it will work to enhance the chocolate flavour even better than hot water.