PSA is the very first layer to staging data in BI , data can be loaded to PSA using Infopack ,
Note : In BW3.x , PSA is optional , you can loaded directly to data target with using PSA , but
In SAP BI , PSA is mandatory , data has to be loaded to PSA first using Infopack and then further to Data target using Transformation/DTP.
Data can be modify at this stage if required and then loaded further to datatarget ( DSO/Cube),
Erronious recored ( faulty record , time overlapped records , for example )can be find directly by selecting the erronious record option in PSA.
PSA capture the data as it is available at source system , So data can be verify/check at line item level ( record by record )
for more features of PSA please search on SDN .