RubyMotion is not a wrapper. It is an implementation of Ruby on the Obj-C runtime. This means that a RubyMotion object is an Obj-C object (for comparison, a Ruby object in MRI is just a C struct). So why would you want to use RubyMotion? Well...
* Obj-C, from the very start, was implemented as a strict superset of C. This means that every valid bit of C code must also be valid Obj-C. This places some rather sever limitations on the Obj-C syntax. Ruby does not have this limitation, and neither does RubyMotion. So one reason you might use RubyMotion is to get a cleaner syntax that does the same thing as Obj-C.
* The Obj-C runtime is very dynamic. You can create classes on the fly, add methods to them at runtime, move methods around between objects, and create methods dynamically. The Obj-C language makes it nearly impossible to take advantage of any of these features of the Obj-C runtime. So another reason you might use RubyMotion is to get the most out of the Obj-C runtime.
* Obj-C, from the very start, was implemented as a strict superset of C. This means that every valid bit of C code must also be valid Obj-C. This places some rather sever limitations on the Obj-C syntax. Ruby does not have this limitation, and neither does RubyMotion. So one reason you might use RubyMotion is to get a cleaner syntax that does the same thing as Obj-C.
* The Obj-C runtime is very dynamic. You can create classes on the fly, add methods to them at runtime, move methods around between objects, and create methods dynamically. The Obj-C language makes it nearly impossible to take advantage of any of these features of the Obj-C runtime. So another reason you might use RubyMotion is to get the most out of the Obj-C runtime.