[atomichaggis]

Bleeding and gutting stop the flesh being tainted from unpleasant flavours. With most fish its not entirely necessary but with some its essential.
Certain Tuna species require immediate bleeding and gutting (removal of bloodline too). Cartilage fish also need immediate bleeding or they will smell and taste like ammonia.
Salmon needs bled as the clotted blood ruins the taste of the flesh, its usually hung from a tree with the gills removed.
White fish like atlantic cod benefit from bleeding and gutting but its not entirely necessary... haddock on the other hand need gutting and cleaning.