Why is Snapping a necessity?

[+Invocation]

In order to gain Allomantic power, you must Snap. Why? Is it because Preservation's influence is so weak that it's latent until your body has to cycle through trauma? Is it because you have to choose to preserve yourself? Is it just plot? Do we know? 

[Calderis he/him]

It's not just Allomancy. It's what "being broken" is in SA as well. 

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Questioner

Why, in your books, are your characters so often, per say - before they get the powers they become broken first.

Brandon Sanderson

There is a narrative reason and an in-world reason. The narrative reason is characters in pain are more interesting to write about. This is just a rule of thumb for writing. Find the person whose in the most trouble, things are going the worst for and that's generally your easiest character. In the stories, the actual Cosmere, the mechanics of the magic finds, this is only one way to describe it - its not the only way - might not be one hundred per cent accurate but it's an easy metaphor, cracks in the soul allow the magic to seep in and that's how you end up with a lot of the different magic systems.

source

Cracks in the soul. If a spiritweb is solid, there's no way for foreign investiture to enter and be used. 

No power from Preservation. No spren bond.

In the case of allomancers, they have the ability from birth. Until they snap, the power can't flow into them, preventing its use. 

[MountainKing]

Do you need to snap to get feruchemical powers?

[Calderis he/him]

16 minutes ago, MountainKing said:

Do you need to snap to get feruchemical powers?

Not that we know of. All of the investiture involved in Feruchemy comes from the spiritweb of the feruchemist, so there's no need for foreign investiture at all. 

[Weltall]

FYI, the in-universe term among the Cosmere-aware for the overarching mechanism is Initiation, which Khriss uses in the 10th Anniversary edition of Elantris' Ars Arcanum.