The simplest reason for the existence of the FCD (Factory Crimp Die) is that some people want to Seat in a separate step from the Crimping step.
Adjusting a single die to both seat and crimp simultaneously is a little more complex than adjusting a die that only does one thing at a time. Also, when you do seat with a combination die, you are applying the crimp while the bullet is still being seated the very last bit. This can cause problems sometimes.
The Seating (the third die) die in the Lee 3-die set has to do both. The Seating die in Lee's 4-die Deluxe set is the same as in the 3-die set, thus can do both, but does not, if you have it adjusted out as David_r suggests.
Thus, when you have the 4-die set, you also have the 3-die set.
Then there is the Post-Sizing function of the Lee FCD. This is more controversial. It ensures that any bulges or out-of-round conditions are ironed out. But it comes with problems. If you squish a lead bullet (inside the brass case), the brass springs back a little and the lead doesn't. Therefore, the grip (friction) the brass has on the bullet is reduced. This is only a problem if your loaded cartridge is, indeed, squished down by the post-sizing. A second problem arises, too. An undersized lead bullet will let hot gasses escape around the sides of the bullet while it is in the barrel. This melts lead from the bullet and lets it deposit in the barrel. Which you will then have to clean out with either harsh chemicals, a lot of scrubbing or some kind of electrolysis. Besides, undersized bullets are lousy for accuracy.
Some people cure the problem by simply knocking the post-sizing ring out of the die. Simple, and irreversible. Others take the care to have their components (bullet diamter and brass wall thickness) sized to prevent any problem that needs such a cure. Micrometer or calipers are a BIG help.
A few people denigrate anyone who uses the FCD.
Do a search in this forum for the thread I started a while back "FCD the virtue and the vice". There were a lot of good contributors to that one.
Thanks for asking our advice.
edit: note that the Lee FCD for bottlenecked cases and the Lee FCD for straight-walled cases are very different. I think you know this already, but others are reading, too.