What is the benefit of using a Merkle Root rather than simply hashing all of the transactions in the block? I understand what the merkle root is and how it is calculated, but I have not been able to figure out what is the actual purpose behind using this kind of hashing solution?

I mean, I know that the root itself must be there, to identify the exact block's payload, but what I don't get is why didn't he just choose to calculate the hash using a normal double SHA256 over the entire block's transactions? Why did he come out with this tree-idea instead? 

I do not see the tree based hashing to be adding any security, neither to have any useful advantages over a regular (linear) hash... so am I missing something?
The Merkle tree allows verifying that a transaction exists in the block without having the entire block, by following its Merkle branch. Among other things this enables:


SPV (lightweight) clients
Blockchain pruning
Smart pool miners