Aggregates tables are important part of OBIA.I have already dicussed about aggregator in my earlier posts "Aggregators".In Datawarhouse we have to there will be many facts and we have to sum up fact data with respect to a given dimension, for example by date(Date dimension).
When we do these summations for each facts it results in slowing down the performance of the mapping. Hence we make use of the Aggregator to do these summations and other aggregate functions like Min,Max,Avg etc.
OBIA pre calculates these sums and stores it in the form of Aggregate tables. In OBIA the aggregate Tables are suffixed using _A. You can easily identify an Aggregate table in OBIA using the suffix.
Also there are many ways using which you can improve the performance of the Aggregator which I have mentioned in my previous posts.