The 'wrapper' technique comes from this example of Esa.
The <div> that is diplayed in [this] info window is coded inline in the page html but not visible on page because it is wrapped in a <div> with style set:
#wrapper{
  width:1px;
  height:1px;
  overflow:hidden;
}
You can put all your info window <div>s in the same wrapper <div>. Just give them unique id-attributes so that they can be referred by openInfoWindow() method.

The benefit is that the info window contents is part of DOM and accessible by getElementById() even the info window is not opened.

The content is also accessible for search engine indexing.