IANAL, but can someone explain what information might be gained via this subpoena to build a case?
To me, it seems like the information from twitter would be useful in building a conspiracy case, but since this guy was only charged with Failure to Obey, Blocking Traffic, and Prohibited Use of Roadway, I don't see how information from his twitter account will help prove those charges. (Unless of course, he tweeted: "I'm blocking the roadway and disobeying police!")
I don't see how information from his twitter account will help prove those charges.
You don't have to. In the US's legal system, the standard for what is discoverable extremely loose. It doesn't just have to be data that you know is relevant, it is any data that might have a chance (even a small one) of containing data that might be relevant.
For example, there's some possibility that the user could have tweeted information about their actions and movements during that time period, and there's some possibility that information might help the attorneys build their theory of the case. So the prosecutors (or plaintiffs, or whoever) are allowed to ask for that information.
It's actually one of the reasons why legal costs in the US and UK are so famously high. The wide net brings in a lot more information that needs to be looked at, and paying attorneys $100s per hour to look at it gets expensive very quickly.
So they can keep him in jail for an extra couple hours by charging him with "Conspiracy to fail to obey, Conspiracy to block traffic, and Conspiracy to ... uh ... prohibited use of roadway."
To me, it seems like the information from twitter would be useful in building a conspiracy case, but since this guy was only charged with Failure to Obey, Blocking Traffic, and Prohibited Use of Roadway, I don't see how information from his twitter account will help prove those charges. (Unless of course, he tweeted: "I'm blocking the roadway and disobeying police!")