So that we can predict whether a given reaction is spontaneous, given (delta)H and (delta)S

G = H - TS

For isothermal: (delta)G = (delta)H - T (delta)S

If we find that (delta) G is:
negative - reaction spontaneous
zero - reaction @ equilibrium
positive - reaction nonspontaneous, but spontaneous in opp direction

In a spontaneous process, the free energy (G) always decreases, given constant temperature and pressure