Why the day matters
Arrears cases are won and lost on dates. If your record says the rent was late "sometime in March", you are relying on memory. The ledger records the due date, the amount due and what actually arrived, so the arrears figure has a date attached to every part of it.
Part payments
A partial payment reduces the balance rather than clearing the month. The amount still owed is tracked explicitly, which is the number that matters when arrears are being counted against a threshold.
The contact trail
Reminders and messages are logged against the property and the tenancy, so the sequence of what you did and when is recoverable later without going through your sent items.
If it escalates
Ground 8 is a rent arrears ground with its own threshold, and the notice drafter reads the ledger rather than asking you to type a figure in. Get the recording right early and the notice is a formality; get it wrong and no amount of paperwork later fixes it.