>>112890668That's because it was the first one in the series. It turns you into a huge, nearly invincible, gigantic ape and boosts your power tenfold, but without training you can't control it and go feral and merely cutting off the tail will make the transformation impossible for some reason. NuBroly somehow managed to tap into the 10x power boost, and a bit of the feralness, without transforming at all and without a tail, which is technically more useful but we've only seen him do it.
The reason the saiyans even used Oozaru and thought it was the pinnacle of their strength is because it's as easy as looking at a planet's moon and their entire race can do it. The Super Saiyan transformation makes it completely redundant but, before Goku came along at least, was so fucking rare it was a legend and one hadn't been seen in a thousand years at that point. Hell, upon seeing Vegeta effortlessly transform into a super saiyan, Paragus is completely fucking shocked and nearly shits himself.
>>112890754This. We needed a way for Broly to keep up with Goku and Vegeta without outright going SSJ right off the bat, so they gave him an admittedly kind of cool berserker ability.
