Adding my own answer after I`ve learned a lot more about usage of passive rectifiers:
2) In normal operation I assume its diodes also works as passive
rectifiers. With its transistors are only used for boosting input
Yes. Transistors are present mainly for boosting DC voltage. This gives several benefits.
Partial load control
If there is a malfunction on the rectifier side (high temperature, low frequency..etc) it could stop boosting, which in turn drops DC voltage. If consumers of the DC link are all inverters, they can have an undervoltage limit, and stop consuming.
After of which the system might recover, and rectification can continue. If not, supply can be broken without current, which is much easier on the breaker.
As vini_i mentioned this will not help if the DC link itself is shorted.
Increasing DC link above rectifier levels means the booster controls when the current should be drawn. This means current spikes when AC voltage peaks can be completely avoided, and draw pure sine waves of current. This means less transients and THD on the supply network.
Power factor compensation
If you can control when to draw current, you can also control the phase of the current compared to supply voltage. This in turn means you can have almost any desired power factor. Which generally means lower overall current, less I^2*R copper losses, and in turn higher efficiency.
Quote: vini_i: The MOSFETs are there not to boost voltage but to get better
The part about efficiency is still true. If the transistors are MOSFETs they could be used as a more efficient diode. Generally called a synchronous rectifier.
This is NOT the case for IGBTs however, since they can only conduct in one direction. They are also usually the transistor of choice in 690V applications as mentioned in the original question.