Compact - with a laptop, everything is in one spot. Your mouse, keyboard and monitor. Unlike a desktop where you would need to buy them separately.
Mobility - you can take your laptop anywhere.
Convenience - just like how I stated before. Everything about your laptop is in one spot. You can do your work while watching a tv.
Cost effective - when a laptop says that it's like a certain price, for example $999, then that is the price. You wouldn't need to go out and buy anything else, like a keyboard, mouse and monitor.
The extras might not be important unless you need it, like a bag, antivirus software, and office software. But that's the same with desktops as well.