It's a code free tool (however if REQUIRED coding can be done) - THEREFORE it helps functional people to upload data from legacy to SAP system.
Since it's an uploading tool provided by SAP, it has a structured way of working. You will have to finish EVERY step before starting the NEXT step.
Since it is integrated in R/3 it is independent of individual platform. The entire project can be downloaded and UPLOADED in any environment unlike BDC which has to be developed in development server and then transported to Production Server.
It can be preferred when there is a need to upload large volume of data.
It is preferred only if there is a requirement for one time transfer of data.