basically, you need some sort of "shielding agent" for the weld in order to prevent impurities from weakening it. The gas helps to keep the weld clean otherwise all kinds of stuff from the air could get in there and contaminate your weld.
One small contaminated part could ultimately lead to that weld failing and not being able to do its job.
If you use solid wire and do not use a a gas, the weld will be full of holes and look like crap. The correct gas will create the proper enviornment for the arc to burn clean and stable to create a strong weld with little impurities. I use Argon for Tig wleding and usually I will get a Tri-mix of c02,argon,helium, for mig welding.
My welder at home uses fluxcore wire only. I am a welder by trade and the fluxcore welder can make me look bad sometimes when doing things that involve low heat. If you want good clean strong welds I would use solid wire with gas.
Flux core is nice if you dont mind some less than perfect welds. Its still strong.
I use my flux core to install subframe connectors and stuff like that. I have never had a problem with it.
i was just thinking, i want this welder for body work ( specific part in mind lower quarter panel hidden by ground fx ), sub frames, exhaust (mid pipe), and little wleding in the engine bay. could i get away using a flux core? im gunna have to sand the crap out of it anyway
a flux core works nice and you dont have to fill a cylinder, but its a little messier because the flux creates a slag similar to stick welding.
gas mig uses usually 75% argon and 25% carbondioxide.
oxygen will burn molten metal, thats why you need shielding, all the gas does it displaces the oxygen, argon is inert and reacts to nothing.
in flux cored the flux just burns as you weld and floats to the top and protects the weld pool from oxygen and contaminants as it cools. and there is a flux left behind, with fluxcored it usually just falls off, but with a stick welder you usually have to chip it off by hand.
if your doing the work inside i'd just get a cheap 110 volt mig welder, since its inside you can use gas shielding and the elements outside wont blow your cover gas away. and the weld is alot cleaner and requires no cleanup. most wire machines you can run both solid wire and flux cored, just have to swap wires and or bottles etc