Immutability
The design of and technology in traditional systems today do not strictly prevent the modification of data. Even in the most critical banking and financial systems, the data can be changed by any participant in its systems. The changes may be controlled, authorized, and auditable, but they require human intervention and are therefore subject to error and even fraud. Adding these checks and balances and auditing multiple systems is a challenging and massive operation in itself that must be undertaken today, with great effort and costs.
The design of and technology in Blockchain systems prevents any changes to the data once it is entered into the system. Change is not possible even if authorized or audited because the technology simply doesn't allow it.
The approach used is that for any transaction that needs to be modified or deleted, another transaction must be created to reflect this change. With this approach, the erroneous transaction is rectified in the overall solution but still stays on record for clear auditing.
It is one of the more valuable advantages of a Blockchain solution, which even traditional systems are not able to offer without extensive customization.