When you say ~ 할 것이다 that means as you already know "will do~" and even if there is no subject, it's expected that someone or something will do some action
However, when you run across ~~~할 것 which doesn't have 이다, so that it can't be used as a verb, but a noun, literally means "to do" , and normaly implies that an action that somebody is planning to do or will do shouldn't be forgotton. For example, when you make a plan, you might make some notes like 운동할 것,to excercise 밥 먹을 것,to have meals, etc., or in a warning sign or instructions, you may be heard to do some action, like 조심할 것, to be aware of danger,
Hope it'll help :P
