I already own physical copies of the Monster Manual, Player's and Dungeon Master's handbooks. Is there a way to get the benefits from the market place like the subraces (that you would get from buying them online) from the physical copies? Or do I have to buy the online version to unlock those features?

Hello Threndor!

This question, and many others, are answered in the D&D Beyond site FAQ. Most important to note is that D&D Beyond is not owned by Wizards of the Coast, and any purchases you have made with a brick and mortar store, or through Amazon, are unconnected to the services offered by D&D Beyond.

At this time, all purchases on D&D Beyond are for content on D&D Beyond only. There is not a way to unlock the digital content if you have purchased the books. We realize and understand the concerns over re-purchasing content, but the material in D&D Beyond is in a different format and platform. 

There are many options provided by the Marketplace that allow you to buy just the material you need one at a time, and the purchases you make decrease the cost of the full book on D&D Beyond by the amount you spend, so you can chip away at a book until it's free!

We also have a robust homebrew system that utilizes the same tools that the D&D Beyond team have used to enter in the official content. You are welcome to use these tools and your physical books to create the content you wish to use. It's a little more effort, but is completely free! 

I hope that this information is helpful!

I do really love the homebrew here! I use it every day.

This gets asked a lot.

No, having the physical books does not mean you can get the D&D Beyond versions for free.

D&D Beyond is a different company than Wizards, which still has to pay a licensing fee for the books.

See this thread for more information and the value you get