In My experience, if a sub is open to wearing panties and the Domme orders it to be done, it is a constant reminder to the sub of his place and shows his obedience. I personally am not into the whole sissification thing because it does not really do anything to turn Me on, BUT, I do realize that for many subs, that bit of humiliation feeds right into their desires and makes them so much more submissive. And they are also a bit of sensation play with their silkiness. I had a sub in panties last week and I loved teasing his cock through them, making him soak them with his precum.
There was a little bit of danger that he could be caught wearing them when he went to the men's room at My order, at the bar we were at, and that also heightened his excitement. I also put him in a pair of garter stockings because it made his ass look so much more fuckable and I knew it was turning him on tremendously.
That ultimately gives Me more control over him. It is a cheap and easy way to assert control in a secretive manner while out in the vanilla world or when he is not in the Domme's presence. A cock cage is more expensive and a bit harder to hide. No man is going to flirt with another woman if he is wearing a pair of silky panties. It's ultimately about the control.
Hope that helps