quasi bidirectional I/O

[banh]

from: https://en.wikipedia.org/wiki/Quasi-bidirectional_I/O

A quasi-bidirectional I/O is a type of input-output port on an integrated circuit such as a PIA. It can be used as an input or output without the use of a control signal for data direction. At power-on the I/Os are HIGH. In this mode, only a current source to VDD is active. An additional strong pull-up to VDD allows fast rising edges into heavily loaded outputs.

what is the advantage of having quasi-bidirectional I/O for the MCU?

 

[VVV]

As you can see, these are really open-drain outputs.
Therefore, you can actually short them together and have a wired-OR circuit, when necessary. This could be an advantage in some situations.

Then, you do not have a direction register to worry about. Whenever you want to pull the pin low, you just write a 0 to the pin, without having to also write the direction register, thus it can save an instruction cycle.

You can easily use one such line as a bidirectional communication line between two similar micros. Since the pins are open-drain, you can thus short together the pins of two micros and transmit data serially in both directions, without having to worry about direction. The most that can happen is data collision, which the SW can rectify.
There are in fact some chips which require such a communication line. An example is the temperature sensor DS18S20 from Maxim. Thus, it is easy to interface to such a device.

On the other hand, you do not have a strong driver, just a pullup, which means you generally cannot source significant current from such a pin, so you cannot directly drive something that draws significant current. In some cases, this is a disadvantage.