I've got a pressure barrel, but it's only got the standard cap, not the one that allows Co2 injection. My question is, What exactly does the Co2 do, apart from force the beer out? Does it add fizz to the beer at all, or is that taken care of in the secondary ferment? (I don't like fizzy beer at all anyway) Sorry for the newbie questions, but......I'm new at this!....
I've spent months trying to get the CO2 question right. But then I've spent decades getting the CO2 question wrong! So I'm a bit opinionated on the subject.
Do you need CO2? Well you can get by without it... but. Yes, you probably do!
We use CO2 because that's what yeast produces. And yes it can make your beer "fizzy" like "pop" ("soda" for American readers). You can get a tank of CO2 and make your beer even fizzier! Or better, have a tank of CO2 and carefully maintain a preferred level of "fizziness". Using yeast (secondary ferment) to
maintain a preferred level of fizziness is tricky, and over a period your beer will range from very fizzy to "it wont come out of the keg" 'cos there isn't enough gas pressure (you can let air in to get the beer out, but the beer goes off because the CO2 was keeping the bugs out).
In Britain if we had a low tech "household" method of dispensing beer we've forgotten it or lost the taste for it. The nearest we've got to low tech dispensing is so-called "Real Ale" in Pubs. Doesn't help us home-brewers because the "Real Ale" method doesn't work more than a few days before any remaining beer goes off.
CO2 in "Real Ale" is enormously important, you must not think of "Real Ale" as flat (devoid of CO2). Beer without any dissolved CO2 is nasty stuff (ever taken a swig out of a glass of beer you didn't finish the day before). "Real Ale" is saturated in CO2 due to the yeast's activity, and a bit more CO2 because the traditional "venting" process doesn't (must not) leave beer devoid of excess CO2.
So yes, extra CO2 is essential. Even for a die-hard "Real Ale" drinker. I'm currently using variable propane regulators (50-150mbar) to maintain the right ("Real Ale") level of CO2 (a normal 37mbar propane regulator, or a 0mbar cask breather, will not help for long).