AIS Java capabilities

Hello,

I can not seem to find clear documentation on this.  What is the difference between using JDeveloper to create a java class using AIS_JAVA.Jar capabilities, and using the pre built AIS REST capabilities (Http:MyServer:8080/jdeRest/v2/formservice)?  To me they seem to have the same capabilities?  For example I want to call a JDE business function.  To do that with the built in REST capabilities, I would make a form execute execute that business function and call that form.  My assumption with creating a java class is that I could just call the BSFN without making a form, but from what I am