Many of the comments seem to be along the lines of "why create a markup syntax to build more markup and throw it in JavaScript" - In my opinion that's not the real benefit of JSX, but just one use case.

The great thing about JSX is that it doesn't build markup. It doesn't even need to be used for markup, it can be used for anything. It's frequently associated with markup because it has been used alongside React, but JSX standalone is a simple syntactic transform for XML-ish notation to JavaScript.

This is quite convenient in some cases, as you can elegantly write DSL's, where each of the tag names are JavaScript functions - and writing the equivalent JavaScript would look something closer to a lisp (moving the function name to the right of the paren).

This:

  <Database name="business">
    <Table name="user">
      <Column name="id" type='int' length={10} />
      <Column name="first_name" type='string' length={255} />
      <Column name="last_name" type='string' length={255} />
      <Column name="choice" type='enum' values={['a', 'b', 'c']} />
    </Table>
    <Table name="account">
      <Column name="id" type='int' length={10} />
      <Column name="first_name" type='string' length={255} />
      <Column name="last_name" type='string' length={255} />
      <Column name="choice" type='enum' values={['a', 'b', 'c']} />
    </Table>
  </Database>

run through the compiler[1] becomes this:

  Database({name: "business"}, 
    Table({name: "user"}, 
      Column({name: "id", type: "int", length: 10}), 
      Column({name: "first_name", type: "string", length: 255}), 
      Column({name: "last_name", type: "string", length: 255}), 
      Column({name: "choice", type: "enum", values: ['a', 'b', 'c']})
    ), 
    Table({name: "account"}, 
      Column({name: "id", type: "int", length: 10}), 
      Column({name: "first_name", type: "string", length: 255}), 
      Column({name: "last_name", type: "string", length: 255}), 
      Column({name: "choice", type: "enum", values: ['a', 'b', 'c']})
    )
  )

I find the first easier to read / reorganize in blocks, which can be useful in lots of situations... definitely not all situations - but having declarative markup while being able to imagine it executing as the latter, constructing actual JS objects with specific logic built in - rather than needing to "parse" the XML tree and deal with an extra step of execution, etc, etc.

[1]: http://facebook.github.io/react/jsx-compiler.html