A very high fever can kill you in a fairly short amount of time in pretty nasty ways. You can go for weeks without eating. If you are staying hydrated and have no antibiotics or antihelmetics or anti malarials etc the slight increased calories (really, did you do the math?) is a no brainer against whatever immune benefits you get from killing off what is likely the major source of death until recent times (infections). This is very much forward momentum in the survival game.