This had been described already in Exodus 21:12–14, as well as Numbers 35 and Deuteronomy 4 and 19. Exodus 21 places the law of asylum at the head of its discussion of capital offences. It describes how God will designate a place for the unintentional killer to flee for safety. Numbers 35:9–15 defines six places as towns of asylum, three east of the Jordan and three to the west. Verses 22–28 go on to state that the town must guarantee protection for the person who is found not guilty of murder, but if the person wanders from the town he may be killed by the avenger of blood. Deuteronomy 4:41–43 describes the three towns of asylum east of the Jordan which Moses designated in that area.

Richard S. Hess, Joshua: An Introduction and Commentary, vol. 6, Tyndale Old Testament Commentaries (Downers Grove, IL: InterVarsity Press, 1996), 305.

1. The Lord spake unto Joshua … Appoint out for you cities of refuge—(See Nu 35:9–28; De 19:1–13). The command here recorded was given on their going to occupy their allotted settlements. The sanctuaries were not temples or altars, as in other countries, but inhabited cities; and the design was not to screen criminals, but only to afford the homicide protection from the vengeance of the deceased’s relatives until it should have been ascertained whether the death had resulted from accident and momentary passion, or from premeditated malice. The institution of the cities of refuge, together with the rules prescribed for the guidance of those who sought an asylum within their walls, was an important provision, tending to secure the ends of justice as well as of mercy.

4. he that doth flee unto one of those cities shall stand at the entering of the gate of the city—It was the place of public resort, and on arriving there he related his tale of distress to the elders, who were bound to give him shelter and the means of support, until the local authorities (Jos 20:6), having carefully investigated the case, should have pronounced the decision. If found guilty, the manslayer was surrendered to the blood-avenger; if extenuating circumstances appeared, he was to remain in the city of refuge, where he would be safe from the vindictive feelings of his pursuers; but he forfeited the privilege of immunity the moment he ventured beyond the walls.

Robert Jamieson, A. R. Fausset, and David Brown, Commentary Critical and Explanatory on the Whole Bible, vol. 1 (Oak Harbor, WA: Logos Research Systems, Inc., 1997), 155–156.

One of the first ordinances after the announcement of the Ten Commandments provided for the future establishment of cities of refuge (Ex. 21:12–13). These cities, providing havens for unintentional manslayers, are discussed in detail in Numbers 35:6–34 and Deuteronomy 19:1–14. The present chapter discusses their appointment after the Conquest (see their locations on the map “Canaan in the Conquest” near Josh. 3).

The fact that these cities are discussed in four books of the Old Testament marks them as being of great importance. It is apparent that God wished to impress on Israel the sanctity of human life. To put an end to a person’s life, even if done unintentionally, is a serious thing, and the cities of refuge underscored this emphatically.

In the ancient world blood revenge was widely practiced. The moment a person was killed, his nearest relative took responsibility for vengeance. This ancient rite of vendetta was often handed down from one generation to another so that increasingly larger numbers of innocent people died violently. The need in ancient Israel for the refuge that these special cities provided is evident.

Donald K. Campbell, “Joshua,” in The Bible Knowledge Commentary: An Exposition of the Scriptures, ed. J. F. Walvoord and R. B. Zuck, vol. 1 (Wheaton, IL: Victor Books, 1985), 362–363.

The need for these cities grew out of the fact that in the ancient world, and to some extent in the Near East even today, there was a custom according to which, if a member of a family or clan was killed by someone, either intentionally or accidentally, the family would gather together and appoint one of its members to be an “avenger of blood” for his relative. This was a world in which the basic legal maxim was “an eye for an eye and a tooth for a tooth.” So if a member of the family was killed, it became the duty of the avenger of blood to track down and kill the murderer. Clearly, there was a certain primitive justice in this system. But a person could be killed by accident, and if that were the situation, it would be an injustice if the avenger were allowed to proceed.

[….]

Once in the city, the frightened man was to appear before the elders, as the text in Joshua shows. He was to state his case, explaining why the death was accidental. Then, if the elders of the city judged that there was no malice aforethought and the death was indeed accidental, they were to admit him to the city, where he was to live in safety. It was necessary for him to remain there until the death of the high priest serving at that time. After that, he could return home in safety.

James Montgomery Boice, Joshua (Grand Rapids, MI: Baker Books, 2005), 108–109.