jade compiles to template(locals) and only instantiate local variables from locals that are defined in the template.
e.g.
```
var title = locals.title
//in the jade 
= title
```
But if we want to pass _app, _model, _parentView etc from rendr, they are access through the 'this' context in the helper, since _app would be defined as local variable since they are not defined in the jade template itself.