With Trump telling his supporters to come to the rally on Dec. 6th because "it will be wild" you never know how much this motivated his supporters to riot at the Capitol. In fact it is sometimes hard to know what motivates his more radical supporters. You might not be aware of what was being posted on radical social media before Wednesday's riot but plans were discussed of capturing Representatives, holding them hostage, trying them as traitors and condemning them to death by hanging. The protester killed on Wednesday, Ashli Babbit, had followed the QAnon posts that talked of taking down the government.
As Representatives took cover in the House chamber protesters try to break down the doors to the Speakers Gallery. If you go through those doors and go to the right you can enter the chamber next to the area where the Speaker sits to preside over the chamber.
Social media posts before the riot had indicated that the plan was to hold the Representatives hostage and conduct a trial that could sentence the Representatives to death unless they voted to overturn the election. This person was seen with ties that could be used to tie up people. It is unclear if this was the intent of this protester but he obviously had an intent to contain someone.