The Lee bullet seating die is also a crimper. You probably are crimping your rounds - - else how is the neck flare removed after seating the bullet?
With the Lee die, the crimp is determined by how far the die is screwed into the die plate, and the bullet seat is determined by the adjustment screw in the top. Adjusting the crimp will also affect the seating depth, adjusting the seating depth does not affect the crimp.
Particularly with magnum rounds, there is a danger that the bullet seating will be disturbed by recoil when other rounds are fired, or by pressure from adjacent rounds in a tubular magazine. Crimping will hold the bullet in place. I have found that the strength/depth of crimp also affects MV (with fast powders at least). More pronounced crimping will increase MV, and shorten case life.
I favour separate bullet seating and crimping, after I noticed that the combined operation can shave lead (with the combined die, the crimping is taking place while the bullet is still moving into the case). So I set the seating die for no crimp, and then use a factory crimp die which is set for no seating.