When you are working from homebrew kits, they come with directions. If you follow the directions and make the beer as intended, not too much record keeping is required. The only thing that you need to write down is the original gravity (OG) and the final gravity (FG) so that you can calculate how much alcohol is in the beer.
Once you graduate from the simplicity of working with a kit, start writing everything down if you want to have a hope of making the beer again. It is also good to be able to refer back to this information to determine "what worked" for future recipes.
My wife and I learned this lesson a few times, but the most memorable one time was with a cider that we made in 2002 (or so). The brewing process basically involved making a Canadian Ale kit as instructed, except we switched out a couple of gallons of cold water with cold cider in the brew bucket during primary. We then pitched the dry yeast from the kit and added champagne yeast to the secondary. I say "basically" because I have no idea how to make this again. We were just playing with the recipe to see what was possible. It never occurred to me that we would end up with something decent. It turned out to be a good dry cider that we would love to be able to make again, but cannot replicate.
I am pretty sure that we used unpasteurized cider because we have subsequently tried to recreate this recipe with both pasteurized cider and cider-concentrate (that we had in the house and needed to use it for something). The concentrate was particularly bad because it did not break down at all. It just sunk to the bottom of the brew bucket and stayed there congealed. The results were awful.
These days, we have the BeerSmith2 software which allows you to play with a recipe for a bit before brewing it. You can also look online and find pages like this one that tells you the Briess Crystal Malt flavor profiles. But, this still does not tell you want the beer will tastes like. The only way to do this is to make the beer, taste it and take notes. This way you can find out what works and what you like so you have a chance to refine your recipes and improve on it the next time.