1. You need speed, speed, speed, in case your hard drive isn't fast (or small) enough.
2. Your MacBook Pro goes through hell daily, taking drops and falls, and you need something more durable than a hard drive.
3. You want your System Information to say "SSD" so that all your friends will piss their pants at the sight of such awesomeness.
4. You want to open up the SSD which contains fairy dust and unicorn blood, which you will drink to become immortal and invincible.