Commentary for Genesis 35:20

היא מצבת קבורת רחל, the one Samuel referred to after the latter departed from him, having been anointed king over Israel. (Samuel I 10,2)

מצבה 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.]