Why are piranhas so important

I have to do a bio project and one of the questions i have to answer is
Why are piranhas so wonderful?

I can't find the answer anywhere, so id really appreciate it if you knew :) thanks!
& please no unneccessary answers please, thanks !

For one they are food for the native people that live along rivers.
Two, they help clean up rivers or lakes because they eat dead organisms.
And that's all I know.

If you need more answers try this: http://en.wikipedia.org/wiki/Piranha