I follow you, Golem.
Adjusting the carb settings is necessary if/when you make any changes to the intake or exhaust. When you allow more air into the engine, and help it escape faster, you have to increase the amount of fuel flow proportionately, or the bike will actually make less power and, in some cases, not even run.
It's all quite simple, even though it can seem like a big project if you don't know what you're getting yourself into.
Don't be mislead by the marketing of these kinds of products. When something calls itself a "Jet Kit", it doesn't have any connection to being fast, like a flying jet or anything.
It simply refers to the parts that are in the kit. You're basically buying a tiny little tool box that contains carb parts; one of them just so happen to be referred to as a jet, which is is this tapered little brass nozzle that allows the fuel to flow from one part of the carb into another.
They look like this:
When I said to adjust your main, needle, and pilot, I was referring to the 3 main adjustable parts of the carb.
The MAIN JET supplies fuel to all parts of the carb, but is mostly used when you're anywhere from 3/4 to FULL throttle.
The needle is this sliding plunger inside that carb that controls fuel flow anywhere from 1/2 to 3/4 throttle. (Most people find a flat spot in the throttle here, so they adjust the needle without any other modifications.)
And the pilot jet control flow from idle to 1/2 throttle.
In that order, you can adjust your carb to allow more fuel to flow. When combined with an increase in air flow, your bike will generate a little more power.
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These bikes are never going to pull your arms off during acceleration. But with some proper tuning, they can run a little better and faster.
Until you start doing any performance modifications, I would focus more on your riding technique and the overall tune of the bike, making sure that everything is doing what it is supposed to be doing. Air pressure, brakes, valve clearance, chain slack...things of that nature.
You CAN, in the meantime, go ahead and lift your needle one notch. That will give you a little experience with the carb, to help build your confidence, and you'll start to see some of this talk put into practice. (There is a nice 'How-To' guide in the instructional articles section)
http://www.gz250bike.com/forum/showthread.php?t=16