Real estate on the web is very similar to real estate in the real world.
When you have areas of congestion you want to make the most of the space you have.
Being able to collapse and expand sections of your web pages can be very useful.
The Accordion technique helps organise your website in a way that looks professional, can save you some precious webpage real estate and reduce clutter.
Like to see how it works? View these examples of collapsible content using the accordion technique: