What difference does Labour’s 'budget responsibility lock' make?

In one sense it makes no difference at all but it does allow Labour to boost its credibility by increasing the reputational cost of departing from its fiscal plans

Labour's manifesto document opens with what it calls its Budget Responsibility Lock:

Despite the capital letters, in one sense, the "lock" will make no difference whatsoever. Labour had already said, as they are now emphasising, that they aim to eliminate the current budget deficit (the difference between day-to-day revenue and spending) as soon as possible in the next parliament, and to cut the size of the deficit every year.

Related: Labour election manifesto: key points