Band and Specs are used on wallbreaker Pokemon. The role of wallbreaker Pokemon is to overwhelm defensive cores with damage output, creating opportunities to sweep. Without Band and Specs, many wallbreakers simply do not do enough damage to create these opportunities. A 50% boost in power is very substantial, and will threaten an OHKO or 2HKO in situations where the alternatives (Life Orb) would not.
Scarf is used on "cleaners" to give the team speed control. It is helpful to have a Pokemon that can reliably come in after a teammate goes down and threaten an attack (because it is faster than the opponent). Without Scarf, cleaners will be outsped any offensive Pokemon with a higher base Speed stat. In general, being faster than your opponent is one of the most important factors in competitive Pokemon, especially in offence v. offence.
But being unable to change attacks is really bad, right?
What beginner players don't understand about competitive Pokemon (at least in singles) is how switch-heavy is it. You are constantly switching Pokemon to help resist attacks, predict your opponent, take advantage of situations, etc. Your opponent does it too. Because of this, the penalty of being locked into an attack is not so devastating, because it leads to one of these scenarios:
- You knock out the opponent's Pokemon with the choiced attack. Fantastic -- maybe that only happened because you had the extra speed or power.
- The opponent survives the attack. OK, sure -- hopefully you knew that would happen and made the right choice. If you picked the wrong move and are now locked into it, that's your fault.
- Your opponent switches out to a Pokemon that matches favourably against your Pokemon. Now, whether your Pokemon is choiced or not, you are in a bad position. You are now likely to switch yourself.
- The opponent switches out, but has no Pokemon that can adequately tank your boosted attack. Fantastic -- maybe that only happened because you had the extra speed or power.
...and having the ability to swap attacks would not change any of those scenarios (except if you picked the wrong move, which is your fault only).
There is a fifth scenario, where your opponent switches to a Pokemon that can take your choiced attack, but couldn't take a different attack your Pokemon knows. It's a shame you now can't use that attack, and have to switch -- but this still isn't a terrible situation for you. If you predict your opponent correctly and choose the correct switch, you can keep the momentum in your favour. The opponent also took 50% more damage than they would've otherwise, which isn't meaningless in a game where chip damage and "putting them in range" is so important.
So, in short: the only situation where being locked into a move has a realistic drawback for you is actually not that bad. Then, the extra power/speed offered by choice far exceeds the inconvenience it causes.
Also, I've seen people add U-Turn or Volt Switch, but if you don't use it on your first turn, isn't it useless? Can anyone tell me why they're so popular?
If the opponent switches out at the same time you use U-Turn or Volt Switch, you will see what Pokemon they switched to before you choose your own switch. That doesn't happen if you switch out normally; it's a blind guess. Seeing what Pokemon your opponent will use lets you choose a Pokemon that matches favourably against it, thereby maintaining "momentum" (i.e., where the Pokemon you're fielding has an advantage against your opponent's Pokemon, so your opponent is under pressure).
Also, if your Pokemon is slower than the opponent's and the opponent uses an attack, then your Pokemon can absorb that attack instead of the Pokemon you intend to switch to. This is a useful way to transition from a defensive Pokemon to an offensive Pokemon, without ever endangering the offensive Pokemon.
