I have several pair of men’s slacks (I’m such a cross dresser) and almost all of them have a button on the inside of the slacks in addition to whatever closures are on the outside of the garment.
The slacks I’m wearing today have an inner button, a metal slide closure (what’s the right term for that anyway?) and a button on the outside. God forbid I have to get my pants off in a really big hurry!
So why’s that inner button there? Does it have a name? Why don’t women’s pants have them?
Men to have bigger bellies. The estra button serves two purposes: to distribute the tension but more importantly to hold the shape. maybe you have noticed men with big bellies and the waist is tight and just below the pants bulge out like an inflated bubble. That second button helps avoid that.
Alternatively, the button could be for the use of suspenders/bracers, but I think sailor has it right.
Does the extra button have a corresponding button hole? Because it might be a spare, in case the regular one pops off or breaks. I’ve seen this on shirts, but never on pants.
The same people who decided that women’s bras needed to close in the back and that their shoes needed really tall heels on them decided that men need to keep their pants from falling down really, really badly, so we get a zipper, a slide closure, a button, a belt and suspenders.
Women, on the other hand, have hips.
Yes, Chronos, it has a corresponding button hole.
JuanitaTech ponders taking off her pants, taking a picture (of the pants, silly) and posting it…
Most of my dress slacks have such a button. (Chronos - yes, there’s a corresponding hole.) Fastening the button makes no difference in how the pants feel to me, but having heard that they can remove a strange “pooch” like sailor describes I felt further investigation was warranted.
I’ve carefully examined myself in the mirror to see if I could tell any difference between having the button fastened or not. Lots of twisting, bending, stretching, etc. I couldn’t tell a damn bit of difference, so I don’t bother fastening it anymore.
I’m a girl and I buy clothes from Layne Bryant (nice clothes for large women) and the slacks I buy there also have the extra button. I suspect because of the belly factor as well.
FYI that extra button and hole comes in handy if you happen to pop off the main button while crawling along the floor! 
It is there to keep the waistband straight, as has been mentioned before. If the button were not there, or not buttoned, a bunching may occur, which would lead to the above mentioned embarassing bulge, as well as causing an irritation to the extreme lower belly of the wearer. It’s probably most common in pleated pants.