Current blockchain based projects are designed to function in a linear sequence; one operation functioning after the other. This is a crucial function so that operations occur precisely in their desired order. This also means that they can be easily added to, and new instructions are not capable of affecting the earlier operations of the script.
There are two things that are cool about this particular data structure.
1) A change in any block invalidates every block after it, which means that you can’t tamper with historical transactions.
2) You only get rewarded if you’re working on the same chain as everyone else, so each participant has an incentive to go with the consensus.