There are two major advantages of hydroelectric power.  One is economic and one is environmental.

Economically speaking, hydroelectric power is rather cheap.  It does require a large initial investment because a dam must be built.  This is typically very expensive.  However, once the dam is built, there are no fuel costs for the running of the power plant.  This makes hydroelectric power typically much cheaper than power derived from other sources.

Environmentally speaking, hydroelectric power is good because it burns no fossil fuels.  It does, of course, change rivers by damming and backing them up.  This can harm fish as well.  However, hydroelectric power does not emit carbon dioxide the way that the use of fossil fuels does.  Therefore, it does not add to the problem of global warming.  It also does not add to air pollution of any sort. 

Thus, hydroelectric power is generally very good for economies and for the environment.  It is not without its problems, but it is clean and relatively cheap.