ERA- isn't some crazy stat. It's just ERA adjusted for park factor and put on a scale where 100 is average so it is easier to compare across teams/players.
WPA, SD, MD are to take into consideration context of bullpen ERA. Cardinals have a better bullpen ERA than the Brewers by 0.50 but that's pretty irrelevant because the WPA, SD, MD better measure how the bullpen is actually doing in leverage sitautions. Brewers have excelled in leverage situations and the Cardinals have stunk it up. Their ERA is better than ours because we have had some huge run totals mounted against our bullpen in non-leverage situations which effectively make no difference in the results of the game.