Why did Yakov find it necessary to build a monument over Rachel's grave? We do not do so for Tzadikim. Their words are their remembrance (Yerushalmi Shekalim 2:5)!
Seforno: Because, since she was buried by the open road, her grave would be subject to becoming broken up and lost.
Oznayim la'Torah #1: Since he changed the name that Rachel gave to Binyamin, by which to be remembered, he compensated her by building a Matzevah over her grave, by which to be remembered.
Malbim, Oznayim la'Torah #2: A solitary grave beside the open road would tend to get lost and people would quickly forget about it. A Matzevah over the grave would ensure that Rachel's burial-cite is remembered forever. 1 Yisrael will need to go there to pray when they are exiled!
Oznayim la'Torah #3: it was customary in those days to bury people in family plots. 2 So to make up for the fact that Rachel was buried on her own, Yakov built a Matzevah over her grave.
As is implied by the following phrase "Hi Matzeves Kevuras Rachel ad ha'Yom!"
Why does the Torah need to stress that "that is the monument of Rachel's grave until today"?
Rashbam: Since Shmuel used it as a landmark when addressing David. 1
Malbim: Other Matzevos are remembered only for that generation. Hers remains eternally; every generation is Mechadesh it, in order to know where she is buried.
See Shmuel 1, 10:2.
It could have said 'Matzeves Rachel'. The word "Kevuras" was not needed!
Peninim mi'Shulchan ha'Gra: These are the only words on the monument. Everyone knew her greatness; there was no need to write more.
Malbim: Other Matzevos are to recall the Mes. Rachel, Yakov's primary wife, would be remembered eternally in any case. She caused that her Matzevah be remembered!"
Why does it say "Kevurasah"? It should say Kevurah! The same applies to "'Matzeves Kevuras Rachel" and "v'Lo Yada Ish Es Kevuraso" (Devarim 34:6)?
Ha'Emek Davar: A grave is called a Kever also before death
