A Repeaters is an electronic device that operates on the physical layer only of the OSI model. A repeater boosts the transmission signal from one segment and continues the signal to another segment. Thus, a repeater allows us to extend the physical length of a network. Signals that carry information can travel a limited distance within a network before degradation of the data integrity due to noise. A repeater receives the signal before attenuation, regenerates the original bit pattern and puts the restored copy back on to the link.

Repeater regenerates the weak signals and make them strong enough so that they can travel its path and reaches to the destination computer or hardware device. Repeater can not make decision, about where to send signals or data. Its work is make weak signals strong.

For example.
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If your cable length is 17 meter between two device. Now your signal strength is only 10 meter. If you want to transmit data between two device without repeater, the signal may become week and receiver will get wrong data. So you need to do is that use the Repeater beteen them.
You can use repeater at 10 meter and 7 meter from both the device. But my point of view you use the repeater at the 8.5 meter from the both device. That will give better perfomance.