I always wondered: why did the legions erect fortified camps whenever they stopped at night? This is a fairly unusual practice than obviously limits the speed with which a legion marches. Is there any incident in early Roman history where one of their armies was destroyed by an attack at night or something similiar?
of course, an army on the march would be vulnerable to all sorts of attack. even worse so when most of the army is asleep :). but there are other negative conditions that a proper camp helped alleviate - like weather, equipment wear, low morale etc.
camps also served as points of retreat and often even as strategic elements - what with various cleverly situated trenches and walls (when battles were fought in close vicinity of the camp).
not all camps were identical though. some were built light ("two day camps", "seven day camps" etc.), some were like small cities ("static camp") with their own money and economy.
so, apart from the obvious military purposes, other advantages involved mainly:
rest and training (incl. hospitals and training yards)
supplies (workshops and numerous compulsory camp duties that focused on building & maintenance, foraging & storage)
and last but not least, resulting from the two points above: order. keeping soldiers busy, motivated and generally in good shape, both physically and mentally.