There are several reasons why Paul may have mentioned the resurrection. It could be a means of indicating that the new age has dawned (cf. Isaiah 26:19; Ezekiel 37:1–14; Daniel 12:1–3) in which God fulfil all saving promises to Israel and to the world. Essentially an initial confirmation that he is writing in the time of the new covenant and that the old has come to an end.1 Alternatively, the resurrection is mentioned to affirm that Paul saw the resurrected Christ.2
1 Paul, an apostle not from men nor through man, but through Jesus Christ and God the Father, who raised him from the dead