I am confused on what rod action is better for what and why.

Is a Moderate tip good for trebles?

Is a Fast tip good for worms and Jigs?

Basicaly, yes.

Read Roadwarrior's article about rod "power" and "action"...

Why:

Moderate or slow action.- the rod bends under pressure acting like a shock absorber

Fast/extra fast.- the rod bends little under pressure allowing you to set the hook

With a bait with treble hooks you don 't set the hook, you let the fish impale on the hook on it 's own.

Thanks for clearing that up.

You are correct.

A medium action rod will act as a "shock absorber" when pulling a crankbait through the water -allowing the lure to be well ingested and securing a good hook set where the hook won't tear out.

A fast action rod is better for slow baits where you need the extra muscle for  the hook set and where you might need to pull the hook through some plastic.  A little stretch is good in many instances so if I'm using a fast action rod, I prefer a monofilament which will stretch some.