Grand jury proceedings are always secret and the indictments are sealed until the defendant has been arrested and formally charged. Grand juries hear testimony from witnesses and receive evidence from prosecutors. You do not want to have the prosecutors show their cards to the defendant when a grand jury has been convened. That would allow the defendant an opportunity to get to witnesses and try to shape their testimony. And prosecutors often have witnesses at this stage who are not known to the defendant.
Once the defendant has been formally charged, the indictment is unsealed and made public. The defendant then has the right to discovery of the prosecutors evidence and identity of witnesses.
This is not a special procedure for Trump. It is how grand juries have operated for decades across the country. Of course, with Trump, everyone pretty much knows what is going on because Michael Cohen was already prosecuted and has basically given up Trump on the hush money scheme.
To give a real world example of why the system works like this, I was involved in a case where an attorney was accused of sexually assaulting his legal assistant. The prosecutor convened a grand jury. Many of the witnesses called were employees of the law firm and personal friends of the defendant and the victim. Had the defendant known who was being called before the grand jury and what exactly where the allegations in the indictment, he could have threatened them with retaliation at work if they testified against him or tried to influence their testimony. And if the witnesses knew what was in the indictment, they could also shape their testimony to support one side or the other.