These new plans will allow you to use more resources (you'll get higher SMS quotas for example) as well as access to certain premium features.

You'll see a link on your Settings page under Billing and Payments to change your plan

This is where you can compare the features available on each plan:

Why not offer everything to all existing users?

With the addition of SMS notifications, Customer Loyalty Tracking, and Image Takeouts, we've realized that some of our users would like to be able to use more of those resources than other users. Since those things cost us money we have to offset that with the higher priced plans.