They say an ounce of prevention is worth a pound of cure. And, this was never more true than when it comes to preventing computer infections. Just think of all the things you're risking with an infection. How much sensitive information are you storing on your hard drive, including work documents, passwords, tax documents, address books, etc.
If you're a Malwarebytes free user, you already know how good the software is at detecting existing viruses. But, what about real-time protection? Malwarebytes free software scanning is manual, meaning you have to purposefully initiate each scan. And, if you are only scanning occasionally (once a day or week), you're likely missing a virus. And, in between scans, a virus might have time to wreak havoc on your hard drive or in other ways collect sensitive/private information to relay back to a third party.
With MalwareBytes Premium, you get active, real time protection against infection. In other words, you're not just scanning to find an infection, you're preventing it.