There are a thousand reasons as to why you may have a pair of suit pants without the jacket. Maybe the pants were on sale? Or they had your pant size but not your jacket size (rarely happens, but anything is possible).
Or last but not least, its just too damn hot to be wearing a jacket. Most men think that if they're not going to wear the whole suit, they might as well forget about getting creative with the pants. Wrong. To me, this doesn't make much sense. If suit jackets and jeans/khakis are a thing, then why can't casual tops and suit pants be a thing?
Smart = not jeans and t-shirt casual, but not so formal that it looks like you're going to a business meeting; it means the sweet spot in between.
Rocking henleys with smart pants (see definition above) aren't just a GQ thing, its something you can start doing tomorrow on casual Friday at the office. The older you get, the more you'll realize that life is about finding the sweet spot. Not too stuffy, not too casual. Not too emotional, not ice cold hearted. Not too needy, not engaged enough.
When rocking a henley with smart pants, your outfit will be hitting the men's fashion sweet spot of not too formal but not too casual.
I won't leave you hanging, below are a few of my favorite henleys on the internet. The length of the shirt is up to you, but your sleeves should be hugging your arms and you'll want to leave as many of your buttons open as you find appropriate. Doing both of these things will make this look more masculine and make you look bigger than you really are (in the good way, not the "I definitely don't track my daily calories" type of big).