not all veins and arteries travel together but most of them do. Reason being that veins don't have good means of pumping up to the heart and this is facilitated by arterial pulsations.
Muscular contractions help venous to pump blood because when muscle contract venous sinuses are caused to contract and blood be pumped up. Where there is no gravity muscular contraction is important because of the veins of the lower extremity.
