Why should we encourage the use of solar energy?
Because it utilizes a source of energy that is otherwise wasted.....
And if we don't use this resource, in order to heat say, water, we have to used coal-fired electricity, which has a large greenhouse footprint. A common application of solar energy is for a hot-water service. Several commercial applications of such systems are available. Of course, these work better in sunny places such as Australia, or Israel, or California, but I have seen a few such systems on the rooves of homes in south UK - I am not so sure that it is quite economic in Blighty, and of course the English never bathe.