Two reasons if you have Android to have a security app...
1) Since Android is "open", malware is more likely to appear on the Android platform.
2) Android on its own doesn't have a built-in "find my device" app.
An app like Lookout Mobile Security fills both voids - checking out for bad apps and gives you the ability to locate a misplaced device.
iphone & ipad users don't generally need a security app because the Apple system is a "walled garden" and bad apps generally don't get into the Apple App Store. These devices also have location apps that help you find a lost iphone or ipad.