It's also embeddable as a library, which means being able to make web-based object customisers: client-side, script-driven tools that don't require CAD knowledge for the user. Like the Thingiverse customiser but on steroids. It's a fascinating project.
And I think it's not the statefulness that is the significant thing about CadQuery and Build123D. It's the access to a bRep kernel, so you can do operations with faces and vertices, you can reflect (analyse, measure) the model, etc.
Being able to do operations on a generated face or edge means not needing to know (or recalculate) the location of that face in 3D space; it saves you so much in the way of maths.
If you have very simple (or very mathematical!) models, OpenSCAD can help. But once things get complex you just have file after file of variable definitions.
Functional flows on vertexes, edges and faces created by previous operations is much closer to a code equivalent of GUI CAD.