There are several reasons to run brand new brass thru the FL die before attempting to load it the first time.
Here is Number 4........ bought some new Jagemann brass for 7MM Remington Magnum.
This stuff looked really nice out of the bag no dented necks so I decided to just Chamfer and load.
Tried to seat first bullet. Would not seat with prettty hard pressure on press handle so after getting the bullet about halfway seated with extreme difficulty I grabbed another piece ...same thing.
They were even harder to get out ruining one bullet.
I have never encountered this before on new brass but it has been a long while since I have not run it thru a die first so maybe these days with QC and quality of brass where it is it's common.

PS I assigned it No.4 but you can use whatever number you like

I agree. I always full length size my new brass, regardless of brand. If for no other reason than to uniform the case neck and mouth, and to save my sanity while chamfering. It's an absolute necessity with brands like Winchester, as I believe they have a position at the end of the line after packaging, whose only job it is to do the river dance on each bag before it ships.

I've quit buying Winchester, Remington, and all those. I started buying only Nosler, Lapua, Norma, or sometimes Hornady if the other three don't offer my cartridge. Not only this, but I anneal brand new brass because improper annealing at the factory is not unheard of. After that I generally neck size, but use FL dies in some instances.

+1

That only fixes your problem if you use an expander in your die. I never do, but to combat this issue (Lapua 6.5x47 is normally pretty tight to) every new piece of brass gets expanded prior to sizing.

Yup...how else could you iron out a dented neck? I use mostly Lee Collet dies and a few Redding neck dies.

A boat tail bullet, and a cheater pipe on the press handle.

Stupid question - how do you not use an expander? Would the neck not be too tight?

Not necessarily. Some guys remove the decapping rod entirely from the sizing die. I've run that way myself with a FL die. The advantage is that the expander button is not there to pull your neck out of alignment with the case body.

I have loaded nosler without sizing. I measure first, but it has been remarkably consistent. Lapua and Norma are top shelf but I still size it. I like and use hornady brass, but it requires more intensive sorting and prep work, with the exception of some 308 match I resized to 7-08. It was surprisingly good.

39 years ago it would have never crossed my mind that factory new brass needed sizing or prep. Of course back then I thought Federal brass was the cat's meow and now I won't even pick it up at the range.

I have one press set up for exactly this

Bushing FL sizing die...my necks only get sized to the size I want them. Closing them up only to expand them is over working the brass. Some very successful shooters do that but with a true expander, not the button expander.