It’s a Russian thing. It is only for the court room. In many countries you do not put someone in a cage while the trial is running, but in Russia the trials can be such a farce that they hold the accused in a small in-room cell to not act up. If you get 10 years in a gulag because you said something against putin, you tend to snap and might want to strangle the corrupt judge as nothing makes sense anymore and everything is just so far away from normality. It also helps the authority that every picture that is taken by the accused automatically paints the accused as a criminal and guilty (he is behind bars right? He must have done something)