Buffer / Line driver ; 3 state IC Working
A tristate output is used when you want to share a line among several devices.
Say for example, you had several serial devices connected together.
One master, and several slaves.
The master could connect its output to the input of all the slaves.
If you connected all the slave outputs together, you would likely get corrupted data back if one was trying talk to the master, and the others had their outputs active, not sending data, but just holding a high or low level.
If the slaves all had a tristate, or Hi-Z buffer, then the inactive slave outputs could be disabled, and only the one actually sending data could transmit cleanly.
The level when all were disabled would likely be a High voltage, with a resistor from the signal to 3.3V or 5V to keep the line from randomly moving around, which could be seen as random High &amp; Low signals at the input of the master, and not valid data coming in.