Essentially, this experiment hides the sidebar's content, but the sidebar still takes up space. Back in 2009, the sidebar was hidden by default and Google made it more visible one year later.
Now that the navigation bar no longer includes many specialized search engines, it doesn't make sense to hide the sidebar. Another issue is that the sidebar highlights advanced options that are relevant to your query, but few people will find them if the sidebar is collapsed by default.
{ via Websonic. Thanks, Akos. }