Junglers and items

I'm starting to learn more about the jungle, but I'm having trouble understanding how to create strategy using the jungle items. I usually only ever use Stalker's Blade, due to my aggressive gameplay and NEED to gank (i need it or i will explode, that happens to me sometimes). But I wonder when would I use an item that doesn't allow the smite to hit an enemy champion?

For example, Poacher's knife seems amazing if my jungle champion is good at invading, but at the same time doesn't seem important late game. On that note, what is it that makes a champion good at invades? Excuse my ignorance, I've never tried to invade so not sure how it works. Please don't just list the champs, I want to know specifically what it is that makes them good at it.

Another item being Ranger's Trailblazer. I can see faster jungle clears with this item... for early to mid game. What about late game? Do I trade it out w/ Stalker's or Skirmisher's? Because by the time late game hits, there isn't much need for an item that lets you clear the jungle faster, you just nuke it all. On that note as well, I took note that Amumu is a champion players seem to favor using Trailblazer on, but he already has the AoE, so what makes Trailblazer better for him than another item?

As far as Skirmisher's sabre goes, I've never used it. Don't know when I should or which champion is good with it or why.

And as for the enchants (devourers, warriors, runeglaive, cinderhulk) please break those down for me. When, where and why would I use which ones with who?

Please help me understand these jungle items!! Again, if you give champion examples, please tell me specifically WHY they are good at the item you're referring to, don't just say "oh Jax is good w/ Stalker's w/ devourer", I want to know WHYYY. Thank you all!!!!!