Wearing leather or rubber wristbands at gay dance and fetish parties looks cool. When you are new to the gay scene maybe you are not familiar with the idea behind it, and maybe you were wearing your wristband on the wrong side for too much time :)
The leather wristband is the modern version of the hanky code. It was very common for gay men wearing colored bandanas on their neck or in the left or right back pocket.
Wearing a wristband on your left wrist means active/top, on your right bottom/passive/submissive and one on both wrists basically means versatile.
Also the color of the wristband is important. A black wristband doesn't mean anything specific, that is the neutral one. In the old hanky code time black used to be S&M. But as leather and rubber mostly come in black this color lost its meaning.
Dark blue means fucking, red is for fisting, yellow for piss and watersport lovers and orange means everything goes.
If you look online there are lists with many more colors. You can start wearing 'fuschia' or 'lavender' but don't expect anybody else to know the meaning of these colors.
Note that many (young) guys are not really aware of the code meaning. Often they wear it because it just looks good with their outfit.
There is an interesting Wikipedia article about the origin and history of the handkerchief code.