I think the purpose was to prevent any kind of protests against Biden during his inauguration, and obviously, this is concerning.
- two weeks earlier the Capital was attacked, for the first time since the British attacked it, in attempt to stop Biden from becoming president.
- they still haven't caught whoever left the pipe bombs
- there has been tons of online chatter planning similar attacks
- Biden, Harris, Obama, Bush, Clintons, Pelosi - all the main villains of every qanon conspiracy theory all in the same place at the same time
Don't worry, since nothing horrible happened there will be plenty of opportunity for people to protest whatever they want.
None of your points are relevant to the concern that the military presence was meant to stifle American's ability to exercise their first amendment right to protest. The United States is not China. The threat of a few extremists committing violence is not a reason to prevent Americans from voicing their concerns. The Chinese government used the actions of a few violent extremists many years ago to suppress the rights of their citizens, including the right to privacy, and to think freely (the violent extremists have been used as the basis for sending millions of minorities to concentration/reeducation camps).