multiple goggles so when you blow up and get snow on the inside of the lense, you can just strap on a new pair of gogs and keep going instead of trying to carefully switch lenses without smudging the anti fog coating.
its nice to have an extra/old pair in your pocket.
multiple suits because you dont want to put 100 days on a $600 goretex suit, when you only really needed it for 20 of those days. it makes shit last much longer. you can wear out the cheaper stuff, and keep the expensive stuff mint for when you really want it.