I used to be into mad arpeggios and stuff and thought I could utilize an extra string to extend runs and such. Turns out it has influenced my writing in a different way, with different chord voicings. The main thing is that I can utilize the low rhythms whilst having all the range of a standard 6er there. The fact is, you can play 6 string songs on an 8 string but not 8 string songs on a 6 string - more options creatively is always a good thing.
Bought mine for the hell of it, but also because the fellas and I were tuned to drop A and my DKMG had absolutely no string tension in that tuning, unless I went for a ridiculous string gauge which meant buzz. I had it down to a choice between a baritone 6 and a 7 and the 7 won out.
My best friend pretty much got a 7 string as his first guitar, so combine that with listening to low-tuned bands and BOOM! Got me a Loomis a while back!