I see this is your first post, so let me begin by saying WELCOME to the DIS BOARDS!
There are valid reasons to purchase from Disney as well as purchase resale. It depends on your situation.
Do you have a favorite
DVC resort that you think you'd like to stay at most of the time? Unless it's Saratoga Springs Resort that Disney is currently selling, you would find the other resorts via the resale market.
Do you need help with financing? Disney tends to work with people who have some credit issues.
Do you need an exact amount of points? With Disney, you can name your point total once you get past 150 points minimum. With a resale contract, you get whatever points the contract holds.
With resales, you can get a contract that is stripped (all points from current year and possibly following year already used) or loaded with banked points. There are more variables there to determine the final negotiated price.
You can save some money by purchasing a resale contract. But it is more involved, takes longer and must be approved by Disney. If the purchase price is too low, they will snap up the contract using their Right of First Refusal and you'll be back to the drawing board.