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:
- 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*.
- 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.
- In Generation I, you don't need to recharge if you KO a Pokemon with Hyper Beam. So it can be pretty awesome there.
- 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.