If we have a straight highway without any horizontal curve, do we need to do a super elevation? If yes, why?

In a straight and flat road,there is no need for super elevation.

no need of super elevation

if we have super elevation it will reduce the speed the of vehicle that is not good

Indeed, no need to super-elevate the road if we don't have any horizontal curve but you should take into consideration that all roads sections even the straight ones shall have a minimum transverse slope for the surface water drainage to catchment areas.