On the surface, it blends APL and LISP. Making S/M-exprs terse and building “diamondless” left-to-right APL on top.
In core, unification of FRP and AD as already mentioned. This is mainly to do efficient online/incremental learning.
Fun and curiosity is the main motivator. Having single system that can do user-editable explorable ML exlanations is just cherry on top.