ANSWER: Jesus, in this well-known parable, uses a group of despised people to illustrate that love and mercy triumphs over biases, hatred and alike. It is interesting to note that in this particular story Jesus never uses the term "good Samaritan," although that is certainly implied (Luke 10:29 - 37).
A little background regarding this parable will help us understand why it was used. Generally, a Samaritan would be an inhabitant of either the city or region of ancient Samaria. They occupied the land formerly belonging to the Israelite tribes of Ephraim and Manasseh. The city was purchased by Omri, the sixth king of Israel (885 - 874 B.C.) and named Samaria after the name of its owner, Shemer. Over time the entire northern kingdom of Israel was also called Samaria (1Kings 13:32, Jeremiah 31:5).
After a three-year assault by Assyrian kings beginning with Shalmaneser V, the city of Samaria finally fell to his army in 723 B.C. (2Kings 17:6, 18:9 - 12). Thousands of Samaria's inhabitants were deported to Assyria. Citizens of the Assyrian cities of Cuthah, Sepharvaim, Ava, and Hamath replaced them. From this point onward, the Samaritans became a mixed people since the poorest Israelites were allowed to remain in the area. Later, Ashurbanipal added a large number of colonists (Ezra 4:9 - 10).
The marriages between the remaining Israelites in the land and Assyrian Gentiles led to the widespread worship of pagan gods. This was the beginning of the animosity between an average Samaritan and a Jew, which is why Jesus used this interplay in his parable.
A historical gap exists in Samaritan history until the time of Ezra and Nehemiah, who returned to Jerusalem from the Captivity around 539 B.C. in order to rebuild the temple. When the rebuilding started, those living in Samaria (enemies of Israel) wanted to sabotage the work through offering help. They became enraged when their deceptive offer was refused. They then set out to do whatever they could to disrupt the work - which did not earn them favor from the Jews (Ezra 4:1 - 5, Nehemiah 4:7).
Manasseh, a man of priestly lineage, obtained permission in about 409 B.C. from Persian King Darius Nothus to build a temple on Mount Gerazim for those in Samaria because they had given him refuge. This enraged the Jews who considered their own temple on the same mount to be superior. Even after John Hyrcanus destroyed their temple (around 130 B.C.) these Jews still worshipped toward it.
The animosity felt by Jews in Judea toward these people continued up to the time of Christ (Luke 9:53 - 54; 10:25 - 37; 17:11 - 19; John 8:48), which is why he used them in his parable to make a point. Their hatred was so great that when they traveled from Galilee to Judea they would bypass Samaria through the barrenness of Petra just to avoid contact. Jesus, however, rebuked His disciples for being hostile toward these people (Luke 9:54 - 56).
Luke 17 gives the account of Jesus healing a Samaritan leper and praising him for being grateful. Jesus also honored a person from Samaria for being neighborly (Luke 10:30 - 37), asked a woman from the area for a drink (John 4:7), and preached directly to them during his ministry (John 4:40 - 42). I hope this helps to deepen you understanding of this well known and fascinating parable.