In theory this may seem unnecessary given if you're in a report suite all data is coming from the same place, however, due to the fact calculated metrics can be used across report suites if you want a single calculated metric with some minor differences between report suites then you have to create multiple metrics. If report suite was a dimension you could use it in conjunction with an IF formula to return a slightly different result for each report suite, that would make it much easier to build a single metric that can work across multiple report suites.
