I also bought an FCD, because when I seated my bullets there was a slight bulge in the brass where the bullet ended. This bulge can be more pronounced on one side of the bullet than another if the bullet is cocked slightly during seating. I thought initially that this was the cause of my extraction problems - sometimes an extracted case would get stuck on this bulge.
What was really the cause of my problem, though, was that the Lee 230 grain round nose bullet has too much of a shoulder on it, and that makes the case mouth stand very proud of the nose of the bullet, and on extraction the case rim of the extracted case would bounce off the nose of the next bullet in the magazine, then catch on the case mouth.
Often it would skip over the case mouth but so much energy would be lost that it would stop on that bulge in the brass. Turns out smoothing out that bulge did not stop the problem because it was not the root problem. Striking the case mouth with the rim was.
What the FCD does is just smooth down the outside of the brass from mouth to almost the rim, restoring it to correct factory dimensions. As dacaur notes, whether you need one or not just depends on how finicky your gun is.
Since I switched to the Lee Truncated Cone bullet, my extraction problems ended, so I no longer use the FCD.