Alden.. different dies do different things... the basic dies are full length resizing , neck only resizing, bullet seating, crimping, and decaping.
Some makes of dies combine some of the above steps Typically decaping is combined with either the full lenght resizing and neck only resizing and most bullet seating dies combine a crimping function.
But sometimes it is more advantagious or preferable not to combine functions.. some folks prefer to decap and then clean thier brass and since you never size uncleaned brass since dirty brass can stratch up the inside of a sizing die and ruin it you may want seperate dies for those two procedures. Likewise bullet seating dies with built in crimpers only offer you one style of crimp , usually a roll or taper crimp depending on manifactuerer or if it is a pistol or rifle round ( pistol uses roll crimp in some applications like for revolvers or rimmed cartridges but taper for semi autos , Rifles use taper crimps , but sometimes you want no crimp with bolt actions ect. ) and Lee makes a "Factory Crimp" die that can't be combined with a bullet seating die because of the method of crimping.
Neck sizing is used exculsively in bolt action rifles and only when you keep the same brass associated with only one rifle .. the body of the brass forms itself to the chamber of that one rifle, you only size the neck for proper bullet holding tension, your brass lasts 10 times longer and you can get better accuracy... you must full lenght resize in any other reloading situation that I can think of.
Carbide dies as mentioned above are typically for straight wall ( usually pistol) cartridges and doesn't require as much lubrication which does add many steps , lubing , cleaning off the lube ect in reloading.. carbide dies can do this because carbie is much harder than tool steel used in regular dies... carbide also used to be much more expensive, but isn't any more .. it used to be a real pocketbook decision to go with carbide and skip the drudgery of lubing.. not so much any more. Some carbide dies even as hard as they are still require some lubrication for some cartridges .. .30 Carbine for example , manifactuers recomend lubing every 10th case of so ..
You have to know what you need to do to a specific case to make it ready for reloading.. if you only have a single rifle.. lets say a bolt action 30-06 and you are only ever going to reload store bought ammo that was first fired in that rifle then you relly only need a necksizer die that can decap and a bullet seater die, with crimping being optional... but if you ever buy new empty brass you would then need a full lenght resizing die to prep new brass for that rifle .. If you had a Garand in 30-06 then you would never need a neck sizing die because all cases would have to be full lenght resized and you would need some form of crimping die because the speed and violence of a semi auto slaming the bullet into the chamber could set back an uncrimped bullet into the case thru inertia and cause a very dangerious over pressure situation.... but if you had a lever action 30-06, say a Winnie 1895 you would need the full lenght resize but not the crimp.
Pistols have some of the same conciderations.
I use Lee dies and I am very impressed with the Lee Factory crimp die.. all most all, but not all ( there are some odd birds out there) brands of dies are interchangeable with all most all brands of presses and there is no rule against mixing and matching different brands of dies when reloading to get exactly the results you are trying to get.
hope this helps