Why might you consider having more than one aggregation table? The short answer is speed and overall resource efficiency.
Consider a heavily used Power BI report where page-load time is considered critical. A typical report may have half a dozen visuals on a page showing values computed over various grains. If the model used by the report has no aggregation tables, all calculations use the raw fact tables to produce values for each metric.
Adding an aggregation table to the model allows the same calculations as before to use smaller tables to produce the same result. Calculations using smaller aggregation tables will enable the server hosting the data model to use much less effort per query.
It was interesting to see just how easy the process is.