They are symbolic. Because the Posse Comitatus Act forbids them from doing anything but watching and reporting. Human drones that cost more and are less efficient. I know I spent a couple of weeks on a Special MAGTF along the border. Miserable time sleeping with Tarantulas checking for snakes every morning and eating crappy food only to watch illegals and smugglers come across the border and not able doing anything about it.
During an Emergency Trump can suspend the Act, but all hell would break loose and justifiably so. You want the next liberal President declaring an Emergency, suspending the Act and using the military to arrest citizens?
The President is being trashed by reporters right now, he is holding his own but the disrespect these clowns show is unbearable.