Exact use of Spring Expression Language

I am trying to learn Spring, so from here I got this example,

ExpressionParser parser = new SpelExpressionParser();

// evals to "Hello World"
String helloWorld = (String) parser.parseExpression("'Hello World'").getValue();

But I didn't understand it exact usage, 'cause I can better declare it as

String helloWorld = "Hello World";

So what is the exact use of above SpEL?

Where can I use it better way?