It's worth noting that within 5e's bounded accuracy, the value of one point of Save DC is substantially higher than the value of one to attack bonus. It's relatively easy for martials to pump their attack bonus high enough that they never really need to worry about hitting successfully, while the means for boosting your save DC as a caster are very limited. Adding one to your save DC (for example by pushing to 18 Int, rather than staying at 16 and allocating elsewhere) is incredibly valuable, comparative to other things you might get for those same points, and in *most* cases is going to be the most appealing course of action, for primary casters.
