Linking areas of a painting
Puddles are brilliant.
Compositionally, they provide a useful method of linking areas of a painting together by bringing colours featured in the top half of a painting down into the lower half.
If your painting is particularly horizontal in structure, then a carefully placed vertical object will help to break up those horizontal bands. For example, a sea-view featuring the sky, the sea and the beach as a series of horizontal bands would benefit from a well-placed boat, where its mast spans all three bands.