The new Apple 27" Thunderbolt display enables you to connect 2 giant external displays to a laptop, something that has not been possible until now without additional hardware.
Large screen real estate has its advantages. It is easier to design presentation slides when you have a large workspace in front of you. Extra space also enables you to open multiple windows, for example a PDF file with comments on the previous version of your presentation, or an Excel file with the data that need to go into your pie chart.
Now, 27" is a lot of space (2550x1440 pixels) and for most ordinary people, one screen will do. A presentation designer might actually need two (putting her in the same category as financial traders, air traffic controllers and social media addicts). I like to design on a clean and calm canvas. All the small windows with bits of information distract me. So I use that second screen as my messy desktop, literally pushing bits, pieces, and windows aside when I do not need them, preserving my pristine and uncluttered design environment in front of me.

Now some technical details. An Apple Thunderbolt screen can only be connected to a recent MacBook laptop that actually has a Thunderbolt port. But more importantly, the dual screen configuration only works on the most recent 15" and 17" MacBook pros, not on the 13" MacBook Pro, and not on the MacBook Air. (This might actually be an argument for getting a MacBook Pro over a MacBook Air) at the time of writing, October 2011).