The longer you perform, the more things you pick up. Some of them are noticeable, while others remain hidden in the shadows.. and by shadows I mean your pants.
If you perform in a dress shirt and at any time put your hands up over your head (or anything even close to it) your shirt can come a tad untucked and stay that way.
At the very least, it can make you look disheveled. At the worst, it looks like hell in still photos. If you ever get the chance, (and I suggest you make the time when you can), talk to some of the old school pros we still have left. They all have their little tips and tricks for keeping yourself looking top notch on stage.
It's a little thing called "Shirt Stays" and they totally do what they say on the package. When you first put them on, it might feel a little goofy at first but it's worth it. Not only does it keep your shirt tucked in, but it also keeps it from wrinkling up from normal wear.
Nice and straight. The shirt at least...
So if this kind of stuff is important to you, then you should look into it. You'll be glad you did.
This could be you.