D&D Beyond is an entirely separate company from Wizards of the Coast. They are just using licensed material from Wizards of the Coast. Therefore, to use the content on this site you have to purchase it here. Hope this clears up any questions you have about accessing the material.
Depending on how you want to use DDB, you might not need to buy entire "books." You can just buy the races/classes/etc. you want if you are primarily looking for a character builder, for example. If you have the time, inclinations, and skills, you can also homebrew most things. The buyer's guide that is linked in my signature details some of the purchasing options.