It survives a pay rise, a quiet December and a month with five Fridays in it.
Why a rate and not an amount
An amount tells you about one month. A rate tells you about a habit, because it survives a pay rise, a quiet December and a month with five Fridays in it. Two months with the same amount saved can be a good month and a bad one, and only the rate can tell them apart.
It is also the number that scales. Raising a rate by a few points compounds through every future month, while cutting one purchase does not. That is the argument for looking at this figure once a month rather than at individual line items.
