There are multiple reasons why ledgers should be immutable. To list a few:
On cancellation of any transaction instead of deleting the GL Entries for that transactions reverse entries will be passed to cancel the effect of that transaction on the date of cancellation.
Since GL Entries linked to a transaction will never be deleted this also means that cancelled transactions and their linked documents can no longer be deleted.
Since the ledgers are immutable now this means future transactions cannot be updated or reposted. So users will no longer be able to post backdated stock transactions.
For Eg: Suppose a Stock Transaction has been made for Item A with posting time as
19-06-2020 23:00:10 then after this transaction you cannot post a transaction for Item A with posting time before this timestamp.