I smoke cigarettes extremely rarely. Mostly, it's a nervous thing to do with hands in some social situations, with the added bonus that you can use it to stall when you can't think of a response in a conversation. I'm pretty comfortable in most social situations, but I feel the need to do something with my hands most of the time. That's true of non-social situations too though I suppose.
Smoking pot is more just relaxation, the same way some people have a couple glasses of wine after finishing with work and responsibilities for the day.
It's worth mentioning that while SMOKING pot is clearly bad for you, at least in the short and medium term (as is inhaling essentially anything that was on fire), vaporizing or baking it into food has truly no established negative effects. There are short term effects of course, but even the decreased lung capacity from smoking (and potentially from vaporizing) goes away eventually. Add to that the fact that there's no evidence of carcinogens in the smoke (though I'm extremely sceptical of the "curing some forms of cancer" nonsense some have bought into), and you've got a pretty harmless substance. Certainly safer than alcohol or tobacco. Hell, though it's become a trite comparison, from what we know both medically and statistically, it's indisputably a safer drug than coffee.