Question

i know game freak wouldn't make them so hard to get if they were useless.

Answer 1

I don't think that they are really hard to get, but often they are expensive to buy. The reason is because they are very powerful moves, even though on average they only have 75 power. There are a few places you could use them:

1. Slaking. Since he slacks off in the turn when you would normally rest, you don't lose anything by using Giga Impact. Plus you get STAB on it. Heck, even his special attack is acceptable enough you
   could run Hyper Beam as well to combat any high-defense foes like Cloyster*.
2. As a last-resort move. If you are low on HP you could use them a bit like Explosion and do serious damage to your opponent before getting KOed yourself.
3. In Generation I, you don't need to recharge if you KO a Pokemon with Hyper Beam. So it can be pretty awesome there.
4. Porygon-Z. Porygon-Z is famous for being able to OHKO a Blissey with its high special attack combined with its ability.

* Most of the Pokemon with high defense are rock or steel types so this may not be a good strategy.

Comments

You can't switch out, the game just carries on without the Fight/Run/Switch options, same as like Outrage.

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This is a great answer. But you were missing one little thing. I'd rather not just have this answer with that one point added, so I hope you don't mind if I added my addition to this.

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Trachy, if the pokemon had high defense, why would you use giga impact or hyper beam (pre gen IV) in the first place? You could just use a different move.

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If it's in the game, you can switch out, but competitively on wifi battles, you won't be able to switch.

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No, my addition was the Porygon-Z part of the answer. Everything else is Pokemaster.

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Giga impact can also see some use on Shedinja, since he has nice attack and won't last long anyway. If you've got good timing, you can do quite a bit of damage before going out.

Answer 2

Giga Impact is good on Slaking.
Both are apparently "acceptable" with Choice items, especially in-game

Comments

Hyper Beam can be good on Porygon-Z, thanks to his high special attack and STAB.

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Yep, really the only two times. In these two cases, they are actually very good.

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Maybe, just maybe with Regigigas, but he's hindered enough as is with his ability.

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I prefer Crush Grip on that guy.