Because he can.
… and what CreekyOne said.
It also ramps up the pressure to plea bargain and (contrary to CreekyOne’s explanation), it can also help to ensure that the prosecution gets something through.
The obstructing an officer charge is a lot easier to prove than the other charges. All they need to show is that a legitimate arrest was being made (whether the charge he was arrested for sticks or not) and that the accused somehow resisted.
