Statues help emphasise particular sections of a garden. They do this by drawing the eye, helping us focus on a particular view by creating a point of interest; a focal point.
The eye is drawn, and without realising it one is looking more deeply at the garden, subconsciously taking greater notice of the environment. Our attention is held, and our curiosity is intensified.
I like to place statues in my garden to give an element of surprise. When a visitor comes across a statue hiding amongst the foliage, it delightfully reveals itself in a playful manner.
I prefer beauty in the garden, so my most important rule is statues should pleasing to the eye and have an element of charm.