Likely there are four stages to brewing.
1) The one can kit, cheap and how most start.
2) The two can kit, better quality, most I am sure progress to this.
3) The one can kit, returning to the one can kit allows one to select different fermentables.
4) The grain.
The grain brewing can also be split up with those who malt their own grain, and those who still use some pre-prepared ingredients.
However I for one have made mistakes, and to work out what the error was you need to alter as little as you can to everything but one part of the process, having a brew fridge so temperature does not change is likely the first step, and using one can kits allows you to experiment changing one thing at a time. I have read kit reviews where some one has changed nearly every thing the manufacturer has said, it may make a good beer, but unless compared to a brew made as recommended by the manufacturer then it is really a waste of time posting the results.
In some cases there is not the room to move to next stage. Or the brewer has decided that's what he wants, so no point moving on. But to compare a one can and two can kit, will likely take 3 months. Until it has been bottled for a few months you don't really know if it has conditioned or not. And using unboiled water things can clearly go wrong, so one brew is not enough you need to do it at least twice.
Some times you just don't realise the error. I have found my brew has way too much head, if I bring in a bottle and slack the cap then drink next day it tastes better, but leave it over 24 hours and taste is going down again. Likely down to amount of sugar used in each bottle, so if I try to correct I first must find a way to measure the sugar better and ensure the beer is not bottled too early. This all takes time. If you are also changing the temperature, and how much hops, and using spray dried malt, then you have not a clue which one made it better or worse.
So we have to slowly move on to the point where we are happy.