Many times we tend to think that if the temperature our fermenter is in is lower than the brewing range called for just giving it some extra time will do the trick, but yeast is an odd little creature, and it works differently that we do. If it is too cold it will actually go dormant rather than just slow down, and in the interim it struggles, kind of like it's sick. This can cause everything from flavor to a stuck fermentation if the wort is too cold.

Many of the flavor compounds are formed during the first few days of fermentation, even the first few hours in many cases, so this can affect the finished flavors of your beer. It also compromises the CO2 layer that forms on top of the wort that keeps the fermentation sanitary and free of possible contaminants. While there is a modicum of heat that is produced from the energy expended by the fermentation process during the first few days, that ends once the peak of fermentation 'high krausen' passes and everything slows down. This can cause your fermentation to stall and delay or prevent completion and yeast settlement.

To ensure a well-flavored finished beer, maintain your ambient temperature within the proper range, rather than on of outside the extremes for the yeast you're using. For the included yeast this is 68-76°F (20-24°C). Other yeasts may vary in their ranges, so you'll want to familiarize yourself with them before you begin the brewing process.