Two dashboards can read the same table and report different conversion rates because they use different populations. A metrics layer defines the measure, eligible rows, time zone, and aggregation behavior in one place. The definition still needs an owner and a version when the business changes its meaning.
Ratios need particular care. Add the underlying numerators and denominators, then divide. Averaging regional rates gives a small region the same weight as a large one unless their populations happen to match. Joins can also multiply measures, so the semantic model must describe relationships and fact grain. Some quantities cannot be added across every dimension: a daily account balance can be summed across accounts for one day, but summing it across days doesn't produce a meaningful current balance. These constraints belong in the definition the serving tools use.