basic - Why should a circuit be grounded?

Why should a transistor circuit be grounded? The way I understand it is that ground is an infinite supply of electrons and so it is equivalent to the negative terminal of a battery. But in this page , there is both a negative terminal and the circuit is also grounded. If I've understood rightly, for a circuit to be complete, we need a positive terminal, some circuit elements & a negative terminal. If grounding is equivalent to a negative terminal, why do we need it in the circuit shown above? (By the way, all the circuits in the book I'm referring were not grounded until transistors started off. You could say that added to the confusion)