Almost any time it makes sense to you, as the table designer. Generally, most people are used to seeing numbers right-aligned. That way they can see the larger numbers, because they are longer and stick out more than the smaller numbers. You can accomplish the same purpose with left-align, but most people are not used to seeing it that way. Center-align might confuse everyone trying to get a quick visual cue about the size of the numbers.
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So that addition can be done without introducing errors (because of non-alignment of columns) and to keep the significance of digit value (position value) of each number in tact and unambiguous