One reason for it is so that Citigroup can't see that Goldman Sachs is buying 500K of some stock and fuck over retail investors by buying up theirs cheaply and

One reason for it is so that Citigroup can't see that Goldman Sachs is buying 500K of some stock and fuck over retail investors by buying up theirs cheaply and selling to GS. They're not evil, just another tool. I don't (and won't) work for them again, but there were non-evil reasons for a dark pool. Anonymous trading (dark pools) also prevents favoritism and collusion too. You're trading based on nothing but the price and public information about the company. So it's less gameable.