שָׁל֥וֹשׁ עָרִ֖ים תַּבְדִּ֣יל לָ֑ךְ בְּת֣וֹךְ אַרְצְךָ֔ אֲשֶׁר֙ יְהוָ֣ה אֱלֹהֶ֔יךָ נֹתֵ֥ן לְךָ֖ לְרִשְׁתָּֽהּ׃
ay magbubukod ka para sa iyo ng tatlong lungsod sa gitna ng iyong lupain na ibinibigay sa iyo ng Walang-Hanggan na iyong Diyos upang angkinin mo.
Mishneh Torah, Positive Mitzvot
To establish six cities of refuge (for those who had committed accidental homicide), as it is said, “Thou shalt prepare thee a way and divide the borders of thy land” (Deut. 19:3).
Sefer HaChinukh
The commandment to prepare six cities of refuge: To separate six cities of refuge from the cities of the Levites, that they should be designated for the one that slays a soul inadvertently to flee there; to fix and straighten the roads that correspond to the cities — and like the matter that they, may their memory be blessed, said (Makkot 10b) that they would write [signs] at the crossroads [saying], “refuge, refuge”; and clear the paths, so that there not be anything on them to delay the fugitive from his run. And about this is it stated (Deuteronomy 19:3), “You shall prepare the path for yourself and divide the border of your Land into three, etc.”
Sefer HaChinukh
The root of this commandment is well-known and clear — that it be so that the inadvertent killer not be killed by the blood avengers.