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-site 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, Yaakov built a Matzevah over her grave.

Rashbam: Since Shmuel used it as a landmark when addressing Shaul. 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.

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, Yaakov's primary wife, would be remembered eternally in any case. She caused that her Matzevah be remembered!"

Ha'amek Davar: A grave is called a Kever also before death - "Ish Mimenu Es Kivro," "b'Kever Asher Karisi" (23:6, 50:5). After burial it is called her Kever also due to the action of burial, which join to the Kever that already was.