What are the uses of transistor in circuits?

Basically each and every amplifier circuit is made up of coupling or use of transistors.

The amplifier amplifies the signals because transistor makes either voltage or current to rise as a result of either of it's regions of operation i.e. open circuit(cut off region) or short circuit (saturation region)

Both the operation regions are not required continuously. Hecne the transistor works in either saturation or cut off regions only when emitter-base junction is forward biased, which can be applied as per requirement.

APPLICATIONS OF TRANSISTOR AND HENCE AMPLIFIERS ARE :

OSCILLATORS, LOUD-SPEAKERS, POWER AMPLIFIERS, VOLTAGE AMPLIFIERS, CURRENT AMPLIFIERS, ANALOG TO DIGITAL CONVERTER, DIGITAL TO ANALOG CONVERTERS