So I've just bought my first aftermarket items for a few of my kits. Metal gun barrels, tracks, etc. I see a lot of detailing kits too. Some of these kits are top notch as they come with everything you could think of. My question is what are these detailing packages? Look like some contain a lot of photo etched parts. I would expect stowage, tools, ammo, etc. for detailing. Most of the higher end kits already come with a lot of photo etched pieces. Just wondering what's left to offer, lol

Depends on what you are trying to create. Lots of guys' work are based on a or a few photos. Photo specific builds rely on specific parts, available from different detail sets.

Your question is very general and would be easier to answer well if you had more specific questions. Generally these sets can be anything, as you mentioned; tracks, barrels, small details.

Some replace original kit parts, some add to the kit without replacing parts, some combine with original kit parts and may require some cutting (surgery) on original kit parts.

from my experience :
detailing is more turned on adding, replacing or correcting details on a kit (bolts, hinges, missing parts, etc.)

tools, ammos or stowage are add-ons in order to give life to a model, like figures