It is to make sure that staff open the till to put the money in and get the 1p change out. Otherwise a lot of £1 coins would get 'skimmed' away and not rung in.
Another reason is to stop assistants from ringing the till with 1p, (if spotted at the time they could say it was a legitimate error) instead of £10 and then remove the balance at a later time.
Also another anti-theft tactic that supermarkets look-out for, is when particular peoples tills are never wrong.
Someone on the fiddle tries to make sure their till is never wrong, thus avoiding suspicion.
Management assume that most people will make a mistake at some time or other, up or down.
Finally (slightly OT)
£xxx.98 = Manager special, possibly scratched etc.
£xxx.97 = Missing parts, such as remote or manual.