I don't know if 'useless' is the right word. My understanding is that a block is simply a page of the distributed ledger (that is, a list of transactions), plu

I don't know if 'useless' is the right word. My understanding is that a block is simply a page of the distributed ledger (that is, a list of transactions), plus a hash preimage problem. (What value do I append to this data, such that the hash function returns some pre-specified value?) That such a computation is expensive to compute (but cheap to verify) is core to some of the properties of cryptocurrency. The alternatives provided by normal data processing are cheaper and faster (if you have trust). If you make the computation cheaper, then financial transactions in the ledger are reversible. But if you make the cheap task expensive, then people will just pay for regular grid computing because it's cheaper.