Paul opened chapter 15 with an explicit, concise, prioritized description of the Christian gospel, the gospel by which you are saved: Christ died for our sins, was buried, and was raised on the third day in accordance with the Scriptures.
Now Paul turns from defining and affirming the Christian gospel to addressing an aberration of it that had sprung up at the church in Corinth -- a denial of the resurrection of the dead.
An early heresy contended that all things physical were inherently evil, and so it would not be surprising if this had led some to deny the physical resurrection of the dead. What would be the point of reuniting us with our filthy, sinful, physical bodies?
Paul not only addresses this perceived problem, but he also goes on to explain how important the doctrine of the resurrection is. Paul shows them, and us, that you cannot deny one fundamental truth of Christianity and still affirm the rest. To put it simply: without a bodily resurrection, there is no salvation.