Connecting external devices to a PLC controller regardless whether they are input or output is a special subject matter for industry.
If it stands alone, PLC controller itself is nothing. In order to function it needs sensors to obtain information from environment, and it also needs execution devices so it could turn the programmed change into a reality.
Similar concept is seen in how human being functions.
Having a brain is simply not enough. Humans achieve full activity only with processing of information from a sensor (eyes, ears, touch, smell) and by taking action through hands, legs or some tools.
Unlike human being who receives his sensors automatically, when dealing with controllers, sensors have to be subsequently connected to a PLC.