Data mart is based on a concept that all KPIs share similar dimensions (some might not be relevant).
Analytical facts provide measures for benchmarking actual vs. target or budgeted vs. month ago vs. year ago.
Measures are pre-calculated for data consistency when data is analyzed by various combinations of dimensions. This way, "as of date" and "average" measures show consistent values.
If a customer decides to start with KPI engine as the first BI project, it is important to account for data sources (typically, if KPIs cover most of what company does, then KPI calculations need all of the corresponding source systems that register those actions).