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human
[DBZ]Why would Goku and Vegeta ever need to use the Potara earrings when they can just fuse whenever they want into Gogeta who is just as strong?

Is there some limit to the fusion? Am I missing something? I’ve read that both Gogeta and Vegito are more or less equal and power, so it seems to me they should just fuse whenever they want and maybe let someone else use the Potara earrings.
gpt
There's a difference in power levels between Gogeta and Vegito. The Potara fusion method will generally create a much stronger fused being, so long as the two people are compatible enough. Because of this, Vegito is a much stronger fusion of Goku and Vegeta than Gogeta. 

The Potara earrings fusion method also lasts longer than the fusion dance method. When they were first introduced, the Potara fusion method lasts for the rest of their lives, except if there as some kind of outside influence. This is why Goku and Vegeta didn't turn Super Saiyan prior to Potara fusing--at least in the manga continuity, it would have shortened their life.

Potara fusion is also less of a power drain than the fusion dance. Because of that, it gives the fused character more freedom to go all out with their kai attacks, even if those attacks may have been a last resort if they used the fusion dance.

The participants don't need to have the exact same power level to do it. They can just fuse. This means that the power increase is based on the highest power levels of both characters, not of the power level of the weaker character. It also means they're not wasting time waiting for a failed fusion to wear off because they got the dance wrong the first time.

In terms of the actual circumstances of the fusion, consider where they were at the time. Buu had already killed everyone else on Earth. The only survivors had been Dende, Hercule, Tien, Piccolo (who'd since been absorbed by Buu), Gotenks (who'd since been absorbed by Buu), and Gohan (who'd been absorbed almost immediately prior to Goku and Vegeta fusing). 

Buu was at his absolute strongest at this point. You wouldn't have the time to wait around for ninety minutes because you fucked up the fusion dance the first time (half an hour for the fusion, an hour before you could fuse again). You also didn't want a method that Buu could disrupt.