Intentional introduction is when humans purposefully introduce species to an environment. There are many reasons that this is done. It can be done for biological control, or economics (such as fishing and hunting species). In ffact a lot of the food we eata re invasive species; they were brought over from places other than the United States. Many times these species cna become invasive.
One reason why this is done is biological control. Biological control is the use of organisms to try to control other invasive species. For example in many areas infected with cane beetles cane toads were intentionally introduced to try to keep the beetles in control (1).
(1) Southwest Biological Science Center. (13, April 15). Retrieved April 13, 2015, from http://sbsc.wr.usgs.gov/products/htms/toad-frog.aspx