There are many benefits in having accurate control over the temperature of your brew. It allows you to control many variables. You can:
- control the ester profile
- alter the speed of fermentation
- improve the health of your yeast
- speed up the clearing of your brew
Lets look at 1, for the first 3 days you get the majority of your yeast reproduction, fermentation and thus ester production occurring. A slightly higher temperature here can lead to a more ester rich flavor profile, due to more rapid yeast growth; conversely a cooler temp will lead to a cleaner flavour.
This leads on to 2. faster yeast growth leads to a brew reaching the end point of a fermentation faster. A lower temp can lead to a cleaner flavour, but a slower fermentation.
Yeast health is affected by the temperature, too hot and you kill it, too cold and it will produce HSP (Heat Stress Proteins) and may fall dormant. Dormancy is usually induced by rapid drops in temp (>1C/hour). Maintaining it in its happy temperature range will help ensure you can reuse the yeast for multiple brews as you have not subjected it to excess stress.
Crashing the temperature at the end of the fermentation can speed the clearing of the beer, and ensure fermentation is halted.
Looking at your Brown Porter recipe, I would personally pitch in at ~20C and let it drop to 17C over the next few hours. I pitch in at 20C as it gives the yeast a little boost to their metabolism as they first encounter the wort. I generally do British ales ~17C as I like the cleaner ester profile most British yeasts give at this temp. At 19C you will likely get a few more fruity esters but nothing that adversely affects the style, as this yeast can tolerate upto 24C you will not get anything drastic, but your brew will likely finish before one at 17C.
With the same recipe and yeast but fermenting at 17C, 19C, 21C, and 24C you will get 4 different tasting beers.
If you wanted a clean flavour but a rapid fermentation, you could pitch at 22 cool to 17 over 6 hours, let ferment for 2-3 days at 17C, then raise the temp to 20 to quickly finish off. Accurate control of temperature and rate of change allows you to tailor your temperature/time profile to your needs.
I hope that helps. For more information grab a copy of Chris White - Yeast