Can you play ID cards by themselves like how you can do Hammers?

You have to have the side-lined character. Otherwise, no reason to use them. The hammers are special objects that could be picked up during game.

If you are asking if you need the round table resource to use the ID cards, the answer is no.
You can place a card on your sideline along with a qualifying character.

The details are on the back of the ID cards.