Ok this subject is perhaps the most basic procedure that many reloader's don't understand.

Brass is a strange metal alloy.  What happens on fireing is that the brass expands to chamber dimentions and rebounds quickly but not to it's original dimentions.  Brass Doesn't  temper like steel but it does work harden and it doesnt respond to annealing like steel.  We will discuss annealing later.

We trim caseses because they grow in length each time we resize them to a narrower diameter.  Material science explains what happens to brass that expands to fit the chamber. Brass rebounds somewhat and this property allows a fired case to be extracted easily .  Over pressure rounds can exceed the elasticity of the brass which makes extraction hard.

Back to trimming
When a brass , bottleneck cartridge expands and rebounds it does not return to its original dimentions.
It gets close, but not quite.

Since it doesn't return to same diameter, we need to resize it.  Resizing involves reducing most dimentions which results in brass growing longer – to explain : making a tube of anything smaller in diameter results in the tube growing longer.  Since we are squishing brass down it must get longer since that is the only free direction case material can flow.
If the brass gets too long it can interfere with the chamber dimentions resulting in an overpressure situation that is dangerous.  What happens is the overly long case gets forced againce the throat dimentions and creates a “crimp” on the bullet which can skyrocket pressures.
This also explains why using full length sizing dies for “neck sizing” is perhaps the least effective procedure loaders can try.  Neck sizing has its place but you must use neck sizing dies in the process.

If you try to use a full length sizing die to neck size a cartridge by backing the die out you end up reducing the shell body diameter before the neck gets sized and you end up moving the shoulder forward and making the headspace measurement useless and/or you will create head separation very quickly.

To clarify : never use full length sizing dies to neck size, you will end up partially sizing the body and pushing the shoulder forward creating a headspace issue.  I may not explaining this well but post your comments.

That said, get the right dies for your style of loading.  Post questions and do it right.

Sorry got to run, I welcome feedback!