BankAccount example. Why do we need balance in domain events?

[pdrolopes]

After completely reading the docs(thanks everyone for the work). I got confused regarding the balance value that is stored in each bank account event that withdraws/deposits some amount of money.

I would expect that the domain events wouldn't contain any balance value and that would be derived after processing all events.
Is there a explanation on why it is a better choice to add computed information in the domain event?

[davegarred]

You don't need it per se, but it is pretty common.
a. Idempotency is an expected feature, this helps with that.
b. Events may be process by many different systems downstream, not all will want to track the current balance, many may not have access to all events.