Posted at the line, not the total
Electronic remittance advice is posted line by line with the adjustment and remark codes intact, and paper EOBs are posted the same way. This is what makes it possible to see that a claim was paid, but paid less than the contracted rate, which a lump-sum posting simply cannot show.
Reconciled against the bank
Posted payments are reconciled against actual deposits so that what your system says you were paid matches what you actually received. Discrepancies get investigated rather than adjusted away.
Underpayments surfaced
Line-level posting makes contractual underpayments visible, along with patient responsibility that was moved unexpectedly and adjustments taken without an obvious basis. Those become work items instead of quietly becoming your write-off number.