Arcgis REST api is the lowest api, it is foundation of all ESRI web map application
All ESRI web based map and application, such as expierience builder, web appbuilder, dashboard apps, instant app, story map, everything you see, is building on arcgis REST api.
arcgis javascript api is higher level api build on top of arcgis REST api.
experience builder is highest level api build on top of arcgis javascript api.
It is like windows api c++ is foundation, c# dot net is higher level api build on top of c++ api.
of course c++ translate to machine language to run on hardware CPU. But the lowest windows api you could use is c++.
That is why ESRI is mostly hiring C++ programmer on Arcgis PRO team.
They do need some c#, but not much. Arcgis PRO is desktop application is made of c++ api.
why?
Sometime, higher level api does not fully implement all function that lower level api have.
More offen, higher level api does not fully document all function that lower level api have.
Today I just found a case that arcgis javascript api(higher leverl) did not document a function that arcgis REST api ( lowest level ) have.
Hash line is implement in both REST api and Javascript api, but did not doucment in javascript api. Maybe they forget to update document it when they implement it in js api.
If you use javascript API, you will never know there is Hash line function because javascript api document did not mention it. Even you know there is, still don't know how to use it, because of no document, don't know how to write it. 
I just use "GUSS" to try without document, it actually works.
arcgis javascript api document missing STYLE property, which is the key to create hash line