quetions for conversion routine in DS

Hello expert,

I have a few questions about PSA:

(1) if there are conversion routine for some fields in the datasource, PSA data is not same as data in the application system. why we need do such conversion in the DS? because I think we can keep PSA data intact from application data source, and do such conversion in BW transformation .

(2) if there are special requirement for the data format, can we create our own conversion routine used in the DS?

Many Thanks,