Why must the trial balance agree?

Trial Balance Agreement means the debit side should match with credit side otherwise something is either missing or wrong information provided.

As we use double entry system in accounting so in every transection debit has equall amount of credit so if the debit side and credit side don't match it means some information is not provided or charged to any account which required to be charged.

For Example:

Purchase machinery of $10000 on cash

[debit] Machinery Account 10000
[credit] Cash 9000

now trial balance will not telly as debit side is more then credit side by $1000 and we can see that why it is so that.