Because everything downstream of it is visible to customers. A wrong rate, a missing line or a duplicated milestone reaches the person who pays you, and correcting it costs a credit note and some credibility.
Check three things: that customer payment terms came across correctly, that the rate on each contract is the rate that was agreed rather than a list price, and that tax treatment is right per customer.
Then run it as a draft and read it line by line against the delivery record. Not the totals — the lines.
Yes, by customer or project, which is the sensible first run.
Fix the source — the milestone or the timesheet — and redraft.
No. Drafting posts nothing.
Half an hour with your own data usually saves reading three of these. The guides will still be here afterwards.