IME you are served a subpoena to testify to specific information that they think you have, and if you can provide it prior to the date of testimony, you don't have to show up.
Suspect a warrant would be if they wanted to take something in your control, but I've never been subject of that kind of thing.
A cellphone would give information about the cellphone. Might be single use/shared burners - might be turned off. In certain cases the Netflix account might give better information (For example, imagine Snowden login in to Netflix in his hotel room. Or more likely, one of the journalists logging in to theirs...).
* subscriber IP address (for an idea of target whereabouts)
* subscriber playback equipment (for confirming target)
* times of account usage (for determining daily routine or pattern analysis, or just confirming that they're home or sth)
* identity info on account or subaccounts (for identifying target or possible friends/family members)
* favorite shows (for chumming up to target to get them to talk/start conversation with)