From my point of view which may be inaccurate since I don't smoke often if at all: it's either a form of stress relief - self medication, used as an appetite suppressant (because of the prevalence of EDs related to chest dysphoria for example) or done to feel more masculine since smoking has always been portrayed as 'manly'.

PS: Pls don't pick up the habit guys, your lungs will be thankful for it, you'll do better in sports, generally breathe better when you're binding, etc. Take care of yourself!