Since I'm not a OS X developer (my skills are pretty much limited to the Terminal and some file system nuances), I have to make a mental wish list of apps I like to see in Mavericks. One such idea was a way to extend the Taskbar area so that items you use less frequently are 'hidden' from view, leaving room for your more important items to show through.
Bartender does just that, with a twist of features that just have me screaming, "Yes! I need this in my life." (No joke - I actually said that when I saw it for the first time.)
If you have a 13-inch laptop like I do, your menubar can get pretty littered with items that can quickly turn your calm OS X layout into a cluster of icons.
Bartender helps me realize there is life outside the menubar. Previously, I would squeeze as many items into the cramped 5-inch space, which started to conflict with certain apps that have numerous menus like BBEdit and Pixelmator. Since I have no control over how many menus are displayed in each app, it was a nightmare when using icons in the menubar which displayed information (like Radium).
Bartender nests items I deem less important into a sub-menu, allowing me to de-clutter my life just a bit more. I can even opt to have the icon show in the menubar when there's a status change, or notification, anywhere from 5 seconds to 10 minutes. This is great for hiding app icons - like Airmail or Messages - and only having them pop back into the regular menubar when it needs to tell me something.
Put simply: Bartender is a great way to keep services, shortcuts and apps running in your OS X menubar when you're short on screen real estate.