In Argentum, you can talk to certain nopon, so he can assess your "Salvager Rank". As for now, all I've found out is that once you advance in rank, you receive some useful items, but that's all. Are there any other uses for certificates?
There is a secret salvage shop in Indol behind a stack of crates on the dock you can only access with the appropriate Salvage Rank. There is more than one salvage shop don't confuse it with the one that is clearly visible.
As far as I can tell, they don't do anything interesting or valuable besides indicate what your salvager rank is.
It looks like there are certain salvager shops that only deal with salvagers of a high enough ranking