From rfc 6962 It is stated that:
Note that the hash calculations for leaves and nodes differ. This domain separation is required to give second preimage resistance.
That means that whenever the hash computes on leaves a distinct known element is preappend to the element $e$: $$H(0||e)$$ and whenever hash applied to parent nodes for leaves $h_0=H(0||e_0)$ $h_1=H(0||e_1)$ then $1$ is being put in the beggining:
It is not clear what is the security implication if $0,1$ is not appended to the hash to separate the two domains. The authors state that this happens to prevent second-preimage attacks. But from Merkle tree hash we require from the hash function $H$ to be collision resistant