Better zoom range and quality (optical zoom instead of just enlarging the image), better low-light capability, image stabilization, better control over the image exposure and focus, probably better battery life (which isn't connected to your ability to make calls), more than likely better image quality overall, more storage capacity with memory cards which are easily swappable, special functions like time-lapse or limited slow-motion capability, better audio quality plus the ability to add an external microphone.
However, there's a saying among photographers: "The best camera is the one you have with you." You can own the best, most expensive camera in the world. But if you don't bring it with you on a trip because it's too bulky or heavy or you just plain forgot it, the smartphone camera is better.
Is there anything in particular that dissatisfies you with your smartphone camera's video? That's probably the best way to judge if you should buy a dedicated camera - if there's something you find lacking or is hindering your ability to shoot video/images.