What is the valence shell electron pair repulsion theory (VSEPR) used for?
To predict the shape of molecular compounds.
Given a molecule, we can determine the number of bonding pairs of electrons, and lone pairs of electrons, fairly routinely; all we need to know is the charge on the species (and it may be neutral), and the number of valence electrons is given directly on the Periodic Table by the element's Group number, i.e. the vertical column in which the element lies.
The electron pairs around the central atom repel each other (duh!), and assume the shape of the Platonic solids: 2 electron pairs, linear; three electron pairs, trigonal planar; 4 electron pairs, tetrahedral; 5 electron pairs, trigonal bipyramidal; 6 electron pairs, octahedral. Now these shapes reflect electronic geometry, but we describe MOLECULAR geometry on the basis of the disposition of atoms, not electron pairs.
And thus water, which has 2 valence electrons from hydrogen, and 6 valence electrons from oxygen, i.e. 4 electron pairs, has an electronic geometry based on a tetrahedron. Its molecular geometry is bent because the