Choice band increases your pokemon's attack by 50%, but it may not change moves until it switches out.
Choice scarf increases your pokemon's speed by 50%, but it may not change moves until it switches out.
Choice specs increases your pokemon's special attack by 50%, but it may not change moves until it switches out.
Most people will put the Scarf on pokemon that are pretty fast, but still have a bit of trouble dealing with major Sweepers. An example would be Typhlosion. With it, he's able to outspeed a lot more threats and catch enemies off guard.
Choice bands and specs are usually given to pokemon that already have high stats in those areas and just want to go for a clean kill. Most of them already have very high speed, so they don't have to worry about outspeeding other things. Choice Band +Outrage is very common since Outrage forces you to repeat the move anyway.
Competitively, there isn't much creativity with these items. Most people just slap one on a sweeper and attack with it. The move torment is a way to counter these guys and force them to switch.
You could also put a choices specs/band on your pokemon and use a move like switcheroo or trick to give it to your opponent; just make sure it's not useful to them. (for example, putting a choice band on Alakazam ruins him since it takes his useful item, and boosts up his attack power, which is abysmal, and the opponent probably won't have a physical move on him. It also forces him to use the same move over and over again)