to improve performence aggregates r used the main use of 
aggregates are

1) when infoobject data is used repeatedley 
2) when master data do not change much
3) to improve the performance in reporting
4) aggregates acts like indexes
demerites :-

1) denormilisation of data in cube and aggregates

if u cube contains infoobject object country we can create 
fixed aggregate on country


      

also save space and speed of accessing data because of 
indexing

To optimize the performance of reporting and analysis 
processes.

Example:

If the client want to see the report of particular customers
(some 8) daily.
In this scenario instead of creating variables we can go 
with aggrecates.