I'm sure that every brewer has heard the term "Controlled fermentation". It's usually one of the most major pieces of advice that a beginner gets, and major improvements a brewer can make in their process. Adding a mechanism (usually a temp-controlled fridge) to control the temperature in which their beer ferments. Controlled fermentation can help a brewer achieve their goals for the particular beer they are brewing. It can completely change the character of the finished product by changing ester levels, effecting attenuation, and helping control by-products.
But fermentation is just one stage where a brewer can exert control to change the outcome of their beer. In fact, almost every stage of the brewing process contains "control points" where a brewer can adjust the actual brew to fit more closely to the planned product.
The ability to adjust on the fly is crucial to the brewer. Many of us, when we start brewing, make beers that don't turn out the way we expect. We use a recipe to plan a beer with specific values: OG, FG, IBU, ABV (don't you just love brewing abbreviations?) all come together in perfect balance on our paper. Yet, when we brew we find that the numbers don't work out right. The OG is too low, the volume is too high, the PH is all wrong, etc etc (ok, so most beginners don't know what PH is, but you get the point.) The same is true when we change equipment: all of the sudden our efficiency calculations are out of whack and out evaporation rate is all wrong.
This last part happened to me lately: I've recently changed roughly everything in my system. I've replace my kettle with a new SS one, which has a different height to surface ratio. I chucked my old mash-tun in favor of a bigger one with a different manifold design. I went from buying pre-milled grain to milling my own. And made a few other "minor" adjustments that basically amount to my not knowing anything about how my system works anymore.
As a result of all this tweaking I have a very hard time designing my recipes. I don't really have my efficiency dialed in yet, so I don't know what preboil gravity to expect. I don't quite have a handle on my evaporation rate, so I'm not sure what water volume to use (I batch sparge, which I think is easier than fly sparge, but does have the disadvantage of needing a predetermined amount of water). And so my "in kettle" results are sometimes quite different than what I expected.
This is where control points become key. I was brewing a Dortmunder a couple of weeks ago - a beer that requires supreme balance between malt and bitterness. My OG was to be 1.056, which meant that I needed a preboil gravity of 1.044. I mashed, sparged, and tested my collected wort. It came in at 1.040, this will not do.
My recipe included a fair amount of Pilsner malt, which meant a 90 minute boil. But the first hop addition was at 60 minutes, which meant that if I did a longer boil, the bitterness would not be effected. And so I did: I brought the wort to a fairly gentle boil (my goal being simply to evaporate the extra liquid, rather than cause any kind of reaction) and took periodic gravity readings. The readings climbed steadily until the hit 1.044, at which point I turned the heat up and called this the beginning of the 90 minutes. I had exactly what I needed.
Of course, I still had a problem with my evaporation rate. Just because I started where I expected to, didn't mean I would end there. And indeed I didn't. 15 minutes before the end of the boil I took a measurement again, and this time it was too high (1.060, I think). Never fear! I was ready with an Erlenmeyer flask full of boiling water at hand (I wanted to add boiling water, not just boiled water, to prevent the temperature in the kettle from dropping). I added a litter of this into the kettle and brought the gravity back down. At the end of the boil my OG read 1.057 - within one gravity point of the recipe plan, and actually closer than I usually got with my old equipment.
Incidentally, brewers can effect their beer even long after brewing: I mentioned here before that I was planning to make a 2.5%. and I did. Except that it fermented out and ended up at 3.8% instead. I didn't mind the extra alcohol, but when I tasted my sample it tasted very thin, and I didn't like that. My solution was to take 100g of lactose, boil it in one liter of water, and add it to my beer (9 Liter batch, so this was actually a substation addition). The result was a beer that was about 3.3% ABV, and with a bit more body than the sample. I also lowered the carbonation a bit (carbonation often causing a lighter perceived body) resulting in a beer that, while not exactly what I planned, was a lot closer to my original vision than what it originally ended up as. I didn't like what I had, so I changed it to match what I wanted. That is the essence of controlled brewing.