Theory : Time is the movement of energy and time exists because energy must move.
Whenever anything happens in time it is linked to a transference or conversion of energy; energy changing form or location.
Perhaps that is because Time IS the movement (transferrence or conversion) of energy.
And perhaps time exists because energy MUST move.
Energy can't remain in the same form or location forever; no matter what form it is in energy will gradually be lost from it, escaping in a different form of energy. For example, if energy is in the form of matter, the matter may not move but it will slowly decay and energy will be lost that way.
So energy is a thing that has to constantly move on and change and perhaps, because everything is made up of energy, so everything is moving and changing and it is that moving and changing we call time and thus the reason why Time exists might be because energy has to move and change.