Road base vs. base course in flexible carriageway

Roadbase is the most significant structural layer in bituminous pavement. It's designed to take up the function of distributing the traffic loads so as not to exceed the bearing capacity of subgrade. Additionally it helps to provide sufficient resistance to fatigue under cyclic loads and to provide a higher stiffness for the pavement structure. 

Though the basecourse is generally provided to give a well-prepared and even surface for the laying on wearing course. Regarding the load distribution function, it also assists to spread traffic loads to roadbase however this isn't the primary function of basecourse.