Git is a DAG where each child of a hashed node is equally valid. For transactions and assets, what we need is a linked list (a chain) and therefore, some rule like "the longest chain is the TRUE chain".
Blockchains are also DAGs. This is the basis of the 51% attack. It is completely possible for a blockchains to fork. The consensus algorithm is how a particular branch is chosen as the winner. But the reason you need them is because blocks fire DAGs.