It helps detecting counterfeit blocks claiming some transactions never happened.
More specifically, it provides confidence that some minimum amount of energy must have been used to calculate that hash. As blocks n+1, n+2, ... are added, the amount of energy that must have been used to create an alternate history of transactions since block n increases. After a certain number of blocks have been added, the cost in energy to forge a longest chain that does not include some transaction (while that transaction does exist on the original chain) will eventually exceed the potential benefit gained by "stealing back" that transaction's BTC. Note that this doesn't guarantee such an attack doesn't happen, it just guarantees such attacks would be uneconomical.
Thus, the "energy waste" of mining is really more like a security limit on transaction amounts and latencies. As the energy used--more specifically, the cost of the energy used--for mining increases, Bitcoin becomes more useful for transferring larger amounts more quickly.
More specifically, it provides confidence that some minimum amount of energy must have been used to calculate that hash. As blocks n+1, n+2, ... are added, the amount of energy that must have been used to create an alternate history of transactions since block n increases. After a certain number of blocks have been added, the cost in energy to forge a longest chain that does not include some transaction (while that transaction does exist on the original chain) will eventually exceed the potential benefit gained by "stealing back" that transaction's BTC. Note that this doesn't guarantee such an attack doesn't happen, it just guarantees such attacks would be uneconomical.
Thus, the "energy waste" of mining is really more like a security limit on transaction amounts and latencies. As the energy used--more specifically, the cost of the energy used--for mining increases, Bitcoin becomes more useful for transferring larger amounts more quickly.