Your basic sizing die is the easiest, most basic, simple way to resize brass to fit back in most chambers.
Don't confuse a simple die resize with the brass being SAAMI specification, or the brass being back to factory specifications, because its simply not.
If a simple sizing die returned the brass to SAAMI specs, then there wouldn't be things like case rollers, ect to return the brass closer to SAAMI specification.
Its your basic starting point...
The next step is usually getting a press that will form the brass consistantly without stretching, deflecting, ect.
When a press/die take on a brass, either can win that show of force, the press usually wins, but not always...
The general rule amongst home reloaders is:
If the ammo fits/fires with little or no issues in your chosen firearm, then you don't worry about going any farther.
Kind of like finding your lost keys, you don't keep looking for them once you find them, so they are always the last place you look...
With some shooters/reloaders, they are looking for hyper accuracy, so no stone goes unturned...
The cases get the works, primer pockets, flash holes, necks trued, cases rolled, ect...
Not all of it works to add accuracy, but you never know what will add accuracy unless you try everything.