they started the grunge trend
Grunge started in Seattle years before Nirvana got famous.
Then grunge went nationwide and Nirvana rode the wave.
Their music is shitty garage music. It's got good attitude and it's sometimes good to listen to. But musically it is very, very, very simple and most songs are not very well put together.
Anyone who's a musician know that Nirvana is just 3 chord rock. Nothing amazing at all. All image and attitude.