Besides being addicted, I mean.
How can quitting smoking cigarettes help someone mentally, given they do not drink or do any other type of drug?
Aside from addiction, nicotine has similar effects to caffeine, so it doesn’t really affect a healthy person mentally any more than a cup of coffee would.
Nicotine and other chemicals like harman and norharman (which act as potent monoaminooxidase inhibitors) in cigarette smoke can normalise brain activity and relieve depression, so you find the vast majority schizophrenics smoke and some people suffering from depression may subconsciously self-medicate with tobacco. Of course, there are less harmful ways of treating schizophrenia and depression, but generally quitting smoking has no positive mental effects.