most elements and all compounds exist as molecules.explain

Every elements in nature need stability. This stability is achieved when valence shell of an atom is fully filled i.e. they have got noble gas electronic configuration. If valence shell is not fully filled, they make it completely filled by transfer of electrons (either gaining or losing ) or by sharing of electrons with other atoms of same type or different types. 

This combination of atoms results in molecule. Hence Molecule is formed for the stability.