Imperial Stout and Champagne Yeast

First all, my apologies if this is a dumb question, but I'm new to the brewing world... Anyway, I brewed an Imperial Stout Extract Kit and am ready to rack to my secondary fermenter. The directions say that I should add a packet of champagne yeast to my secondary. Will this affect flavor? Is this simply done for carbonation purposes? Should I be doing this for all of my high gravity brews? ie O.G. > 1.070? Also, I saw some people mention US-05 dried yeast in place of champagne yeast. What different effect will this have?

Thanks!

I do extract kits, and am a fan of stouts and porters, and have never heard of adding yeast to the secondary. The secondary is more for clarification and pre-bottle conditioning than it is for fermenting.

What is the brand name of this kit?

Edit- Safale US-05 has become my go-to yeast.