The unions get dirt cheap labor, just like the companies they contract with, and get a reliable supply of social-democrat voters to further their cause.
And contrary popular belief here on FR, the majority of hispanics, legal or otherwise, are hard-core leftists, and think Bernie Sanders isn't socialist enough for them. Oh, and most of the hispanics here on the west coast are card carrying la raza/reconquista types, and use the unions to infiltrate companies solely to shut out the “native” population (ie: gringos).
As you saw in Portland, OR. the hispanic and white guilt liberal illegal employer types were falling all over themselves to endorse Sanders.
This needs to be repeated over and over. It cannot be typed or spoken enough.