In addition to the Pro Plan Features, the Designer Plan has some extras.
Rebrickable contains several "Featured MOCs" spots which display MOCs in a prominent location for users to discover. These spots are exclusively for MOCs from designers in the Designer Plan. Any Free MOCs you have will also get promoted, but to a lesser extent (80% Premium, 20% Free). Depending on your MOC's popularity, this can give a boost of around 5-20% in views.
Advanced MOC Analytics
View the last 6 months of data, or you can export the whole lot to CSV files for your own custom analysis.
Your MOC views are separated into "normal" and "promoted" views. If the user found your MOC via one of the various "Featured MOCs" spots on Rebrickable, it is counted as a promoted view.
View your Top Designer ranking over time, to help see the impacts of your MOC submissions.
You can also configure your own Google Analytics tracking ID which will then be inserted into your profile and MOC pages. This will allow you to see the full user session and e-commerce tracking data.
Social Media Promotions
Rebrickable shares MOC submissions via it's social media channels when it is expected they will be popular - at the discretion of the admin team. As a member of the Designer Plan, every MOC submission is guaranteed to be shared.
Normally, shared MOCs are added to a queue which means they get posted within 1-2 weeks of approval. However, Designer Plan MOCs are published within a few hours. You can also configure your Twitter/Facebook accounts to get automatically linked mentions in the posts to help drive traffic to your own social media pages.