Why is the internal resistance of a voltage source made to be low?

When a voltage source, such as a battery or a generator, is on open circuit -in other Words, when it is not supplying a load- the voltage appearing across its terminals is called its 'open circuit voltage' and corresponds numerically to its electromotive force.

However, when the voltage source supplies current to a load, that current also passes through the voltage source itself. This causes an internal voltage drop, which is the product of this current and the voltage source's internal resistance. This voltage drop acts in the opposite direction to the electromotive force and reduces the source's terminal voltage. This internal voltage drop will increase, of course, if either the load current increases or the internal resistance increases.

So, in order to keep that the source's internal voltage drop is as low as possible, its internal resistance must be as low as possible. In the case of a battery, the internal resistance is due to the ionic resistance of the electrolyte/plates, whereas in a generator it is due to the resistance of the windings.