The Hadith explains that discharging the sadaqatul fitr is a means for one to purify his fasts of certain deficiencies that occurred during the fast (e.g. engaging in futile speech). (Sunan Abu Dawood #1609)

The obligation of sadaqatul fitr is not confined to those who fasted in the month of Ramadhaan, but rather it is upon those who fasted during the month of Ramadhaan as well as upon those who did not fast e.g. the ill, women in nifaas, etc.

It is preferable and more rewarding for one to discharge the sadaqatul fitr before the Eid Salaah. However, if one did not discharge the sadaqatul fitr before the Eid Salaah, then one should discharge it after the Eid Salaah.