Sforno on Genesis 35:20/ספורנו על בראשית ל״ה:כ׳

1מצבה על קבורתה, this was for protection against the grave being damaged or exhumed by people doing this for nefarious purposes, as it was located at the crossroads where there is a lot of traffic. [I presume it could not be in a field as Yaakov would not violate private property to bury his dead. Ed.]