It makes it hard to alter historical data. It does not prove the data was correct to begin with. Blockchain isn't the only way to do this but it is a good way i

It makes it hard to alter historical data. It does not prove the data was correct to begin with. Blockchain isn't the only way to do this but it is a good way if you don't trust the parties holding the data not to attempt to alter it. You could avoid that by holding your own copy of the data and checking against that. What solution is best depends on the situation.