Nicotine is used by many people, though usually not consciously, to help focus their attention. Most of us know about chain-smoking writers or baseball pitchers with big wads of chew in their cheeks. This eventually becomes a addictive crutch, especially since (some studies indicate) nicotine may artificially affect certain thought patterns related to concentration. After quitting, you may need a couple of weeks, or even months, to become accustomed to thinking clearly without nicotine. In the long run, your brain will function much more creatively and efficiently without the nicotine 'crutch'.
Heishman SJ. Behavioral and cognitive effects of smoking: relationship to nicotine addiction. Nicotine Tob Res 1999;1 Suppl 2:S143-7; discussion S165-6.