What are exact benefits of using R services on Enterprise vs Standard?
Check out the "Feature availability across editions of SQL Server Machine Learning Services" page of Books Online:


  The difference between basic and advanced integration is scale.
  Advanced integration can use all available cores for parallel
  processing of data sets at any size your computer can accommodate.
  Basic integration is limited to 2 cores and to data sets fitting in
  memory.


Books Online goes into more details on that page about deployment restrictions as well like:


  Only a (Standalone) server offers the operationalization features that
  are included in a Microsoft (non-SQL-branded) R Server or Machine
  Learning Server installation. Operationalization includes web service
  deployment and hosting capabilities.
  
  For an (In-Database) installation, the equivalent approach to
  operationalizing solutions is leveraging the capabilities of the
  database engine, when you convert code to a function that can run in a
  stored procedure.