There is a big world out there beyond rendering.
CAM people, for example drive numerically controlled machines and robots. (CNC machines) These machines are driven by lines, arcs, higher order analytic curves, such as the conic, and recent controllers will accept bsplines.
Having path planning done with real geometric entities brings a lot of benefits along for the ride! Accuracy without a bazillion entities for the look ahead system to manage and for program buffers to drip feed (DNC) to a machine that probably does not have a large enough program memory to deal with that bazillion entities.
In many cases, derivative or extrapolated curves do not convey the right shape or path or are difficult and time consuming to force conformance.
We ended up with the big geometry kernels, such as Parasolid, precisely so that CAD could create a brep (boundary representation model) of sufficient fidelity to make other tasks, such as CAM and simulation, as well as derivative designs possible, practical, repeatable.
Those big code bodies have so damn many man hours of development in them it is very hard to replace it all. And that means we are stuck with some things, like many operations being single threaded, which caps CAD design to be a function of sequential compute speeds.
There are ways to build models to eliminate some dependencies, and the same goes for assemblies, so that multi-processing of some sort is possible. That brings our multi-core CPUs into the game.
But those are hard to teach, a lot of software does not yet support multi-threading anywhere near enough to fully exploit the model work, and, and, and...
In the CAD / CAM world, the primitive is the b-spline, and the surfaces it can represent.
Those generate faces, trim boundaries and sets of those create boundary representation models.
And still those can cause grief. They might be open, or just a surface representation. Or, they may self intersect...
Basically, every edge, which boils down to a b-spline curve that matches up to another b-spline curve serving as a trim boundary of a surface, must be shared by two and only two surfaces or the model is not representative of a real world, manufacturable thing.
Another great observation regarding real world objects is there are no edges, just radii, if only a single molecule big. Just an interesting and perhaps useful aside.
And b-splines are constrained to conform to the properties of lower order entities, such as lines, arcs, and the like. This is a part of why wireframe entities are all called "curves" or "sketch curves." They literally are all b-splines constrained by number and or placement of control points.