Can someone explain to me briefly how is he trying to get mod's personal info and why would it matter?
The users on reddit and discord contributed/participated/organized the copyright infringement and their accounts are named in the lawsuit and they are listed as Doe 1-10 (Jane/John Doe, placeholder for unknown person).
To get their Personally Identifying Information (PII) from Reddit and Discord they require an OK from a judge. This is a normal legal process through which you can request information from a 3rd party, which is relevant to the case. In this case it is the information about the so far unknown parties. In order to properly serve them, Ethan needs to know who they are.
Reddit and Discord are legally required to hand over the information they have about those accounts.
If they were dumb, as you would expect from reddit users, they will likely have used Email addresses that are easily traceable and connected directly from their home or phone Ip addresses which can be directly tied to them.
Once Ethan knows the information all these mods will be served with the same lawsuit, since they are already named in them with the current placeholders Doe 1-10.
Ethan requested the court to allow serving these subpoenas ahead of time, and Denims lawyer agreed, so they filed a joined stipulation by Ethan and Denims.
[EDIT] This is furthermore a big deal because it is very likely the same people had extensive communications between each other and with streamers who also participated in the harassment of Ethan and his family, going as far as making fraudulent allegations and calling CPS on them in an attempt to have their kids taken away, and sending a real human skull to their home.
Once the people are served with the lawsuits, all their communications will be subject to discovery and if information is found about criminal behavior, it will likely result in criminal charges filed against them as well. (In my opinion this is the most important thing of this process, using the civil suits to identify the people who committed crimes)