Why Vipassana meditation need a base object?

I think (please correct me if I am wrong) that Vipassana is about letting go of control and judgement, and the meditator will just become aware and accept things as they are. Be it pain, thoughts, sound, or physical sensations like breath or a moving stomach (as we breath). I wonder we need to have a base object, like breath or moving stomach. Why don't we just let go from the very beginning and just become aware and accept any sensation as it is, without any default object?