I've been debating purchasing additional content to make build for a while now and I'm unclear why I need to buy the Compendium for each Sourcebook...   

I have a number of the physical books and am new enough to D&D that I don't plan to start DM'ing for a while.    So, would it be incorrect to not just purchase the Spell, Race, Class & Sub-Class content alone?

Thank you,  I appreciate any advise.

I am not sure what you are asking for since your question is not clear, but no one is forcing you to buy Compendium Content.

You could just buy the background (PHB: $3.99), subclass (9.99), feat (3.99), race (2.99), spell (5.99), and item bundles within each Sourcebook as necessary (PHB with all the sub bundles except Compendium Content totals to $26.95).

Everything you need for playing D&D (as a regular player or as a DM) is the Basic Rules. And that's entirely free. As a player, who is not DMing, you only need the Basic Rules and what options are relevant for your character sheet if they're not already included in the Basic Rules. It's the DM's job to provide you with everything else - you create your character with your DM so you have a character that can fit into their campaign world and they should provide you with info you need for your character and their backstory.

Some books can make it easier, like if they're setting their campaign in Eberron, having the Eberron book makes it easier to understand that world - however, still not necessary. The DM can fill you in on what you need to know as it pertains to your character.

If you've already got the physical book, you can use homebrew tools on here to recreate many of the character sheet options for private use, for free.

If you have physical and just want to build characters at this time just buy what you need for that.

if you determine you prefer digital and the sites features, just buy digital on new book purchases 

look out for sales and codes to fill in your digital collection 

The “Compendium Only” versions are just digital read-only copies of the books. If you want the interactive tools you will need to purchase the “Full Version” of a book.

Quote from IamSposta >>

The “Compendium Only” versions are just digital read-only copies of the books. If you want the interactive tools you will need to purchase the “Full Version” of a book.

 
Or just those individual options.

Great, Thank you...  I believe I have answer(s) I was seeking.     The non-Compendiums items (race, Spells, etc.) for purchase will unlock anything needed for character creation past the Basic Rules (which are already provided).    And the Compendium itself hold the extra information Lore and possibly game mechanics such as use of Tools and crafting.  

Quote from AkumaHellkite >>

Great, Thank you...  I believe I have answer(s) I was seeking.     The non-Compendiums items (race, Spells, etc.) for purchase will unlock anything needed for character creation past the Basic Rules (which are already provided).    And the Compendium itself hold the extra information Lore and possibly game mechanics such as use of Tools and crafting.  

 Correct. And the “full version” is everything.