The more you know....

The FaceRig Pro DLC is designed for people who wish to earn ad-based money by posting FaceRig generated content online.

The differences between the basic version and the Pro DLC are:

With FaceRig Classic, you are allowed to monetize videos on YouTube/Twitch or similar only as long as the commercial aspect of it is not significant. We consider commercially significant using it on any avenue (channel) that produces you more than $500 monthly revenue. This includes ad-based revenue and voluntary donations (one time or recurrent such as a Patreon). If you are under this threshold you can use Classic, if you are above you need to upgrade to Pro or Studio.
With the FaceRig Pro DLC, you are allowed to monetize videos on YouTube/Twitch regardless of the monthly revenue, but not for content exclusive to subscription based services, such as the new YouTube Red (for content that is exclusive to YouTube Red you will need FaceRig Studio).

Content created with FaceRig Classic that is made public is governed by the Creative Commons license, while content created with the Pro DLC can be made available under whatever license you see fit, as long as it's not being directly sold (see below for unrestricted commercial use - with FaceRig Studio).

FaceRig Pro does not bring new assets, technical features or improvements to FaceRig Classic. If you only want to use it for fun, with friends and family there is no reason why you would need the more expensive Pro DLC, except if you want to further support the FaceRig development effort.