Every invasive exotic (non-native) species has a story of how it got here.
Blue gum and red gum were imported to produce the timber needed for railroad ties. Many annual specie were forage crops for grazing cattle. Some got here by stowing away as seed on or inside cattle or other animals. Supposedly, mustard seed was broadcast by those traveling on the El Camino Real so that other travelers could find the route later. Then there are all sorts of invasive exotics that were imported simply because people liked to grow them in their gardens.
It is difficult to imagine why anyone would import any of the weedy specie of broom (Genista specie) or the sloppy species of pampas grass (Cortaderia jubata). It might have made sense at the time, before more appealing specie were introduced, or before less invasive modern cultivars were developed. Black locust has always been, and continues to be a pretty tree, long after more colorful and less invasive cultivars were developed. They were brought to California by prospectors from the East at a time when no one knew or cared how invasive they would be.
Acacia dealbata was likewise imported simply because it is a pretty tree, before anyone knew how it could naturalize and displace native vegetation and wildlife. Now it grows very rampantly in utility easements where other vegetation has been eradicated. Not only does it interfere with the efficiency of utility cables, but it is also combustible if ignited by sparks from electrical cables. Yet, it is so colorful and pretty in the middle of winter that it is not easy to dislike. Unfortunately, environmentalism is not what it used to be, and some so called environmentalists want it to be protected simply because it is ‘alive’.