The main attraction of watercolours is its freshness and translucence. It is quite a difficult medium to use, as it is quick drying and cannot be overpainted.
One cannot predict how watercolour will behave, especially when applying a wash to create a background. Quite often the painting turns out very different to what was envisioned on starting.
It can also be used effectively with other mediums, such as pen and ink, graphite drawing and acrylic, to create a mixed media painting.