There is a Jewish custom that after visiting a grave one places a pebble or small stone on top of the matzaivah, the monument. It is widely accepted that the reason for this ritual is to let the other individuals who come after you should realize that the deceased was already visited. According to this explanation, when one places a stone on the matzaivah is not for self- benefit to benefit but rather for the sake of others to convey that the departed was cherished by others as well. However, we can glean from this parsha that the placing of the pebble is really to benefit both oneself and the niftar in order to serve as a time for self - introspection and renewed commitment.
In order to understand this novel interpretation we must first must understand a passuk in this week's parsha לא תקים לך מצבה אשר שנא ה' אלקיך Rashi explains that the prohibition of erecting of a matzaivah which is one rock that would serve as an alter to sacrifice even if it is to Hashem. Although we find by Yaakov that he himself made two matzaivos, one upon which he poured oil to Hashem, and the other he made a matzaivah over Rachel's burial place, Chazal say (Sifri Shoftim, Rashi) "What was beloved by Hashem in the era of our Avos became despised in the days of their offspring". Once the gentiles started using the matzaivah for avoda zara it became prohibited by Hashem.
However the question arises why didn't Hashem prohibit even sacrifices to Him for the same reason that the goyim also sacrifice to avodah zarah? Secondly, why wasn't the matzaivah placed over the deceased also prohibited? We can answer this with the yesod of Rav Moshe Feinstein ztl in his sefer Darash Moshe Shoftim where he explains that the word מצבה symbolizes the cessation of spiritual growth. The ladder that Yaakov dreamt of was מוצב ארצה was positioned and standing on the ground. Man is called a הולך and a malach is called a עומד. Malachim don't possess bechirah which explains that their reality is to remain standing in the same place and madraigah of their initial creation forever. They are likened to a matzaiva one rock that nothing is added to it. This is contrary to Man's mission on earth who possesses בחירה free choice from which he spiritually grows by making the correct choices, all in order to receive the waiting reward for his spiritual upgrades in the future world. Note that the first word that Hashem revealed to Avraham was לך go, walk, a directive to change, elevate and perfect one's לך (lecha) yourself.
However there is a difference before and after Matan Torah. Prior to Hashem's giving the Torah to Yisrael it was possible for a Yid to demand from Hashem that he toiled enough in fulfilling the will of Hashem and he wants to reap his reward now. After all the majority of his life until the present time has earned him spiritual miles to deserve early retirement from spirituality and the pursuit of Hashem .He was never given the Torah and its commandments. This option is not available once Hashem gave the Torah and mitzvos. Once Matan Torah occurred one must toil his entire life and only then receive the reward which he achieved during his lifetime. For if he chooses while he is still alive to neglect his mission and waiver the Rewards Program by embracing the compromised status of being be an עומד, he will be punished for failing to be a הולך as commanded. Since Matan Torah we are Hashem's servants forever even if we are willing to forfeit our rewards see Rashi Bamidbar 15'41
אני ה' אלהיכם - עוד למה נא' כדי שלא יאמרו ישראל מפני מה אמר המקום לא שנעש' ונטול שכר אנו לא עושים ולא נוטלים שכר (שבת פח) על כרחכם אני מלככם וכן הוא אומר (יחזקאל כ) אם לא ביד חזקה וגו' אמלוך עליכם
Therefore before Matan Torah the building of a matzaivh was permitted to serve Hashem for then one could have claimed from Hashem to give him his matzav reward from what he accrued. However once we received the Torah it is prohibited to ask for a "cash out payment" and the entire concept of the matzaivh reward by obtaining an early retirement and an extended vacation from the Torah. The only matzaiva that was permitted in this world was at death for it is not prematurely. When one can no longer build up deposits in his Olam Habah account through bechira and therefore he then merits to acquire his reward that was הוצב set aside for all of his avodas Hashem.
Yet the above is not black and white as it seems. For even when the neshama is in shamayim, it can still become a הולך by receiving mitzvos from others. This process is called receiving spiritual pekelech packages from others who perform mitzvos rooted in the deceased's influence, chinuch, or inspiration upon him. As Chazal call a son the כרעיה דאבוה the foot of the father (Eruvin 70b). Why did Chazal choose the foot specifically in order to teach us that the son is an extension of the father? The answer is to teach us that the son after his father's demise has the ability to give to him a foot which symbolizes a person who is classified as a הולך. So even though the father is technically a עומד for one cannot do any mitzvos without his body, nevertheless, his children and people who became motivated to spiritual growth from the departed, can increase his mitzvos and rewards by sending to him mitzvah pekelech daily.
This is the inner meaning of placing a pebble on the matzaivah as if one is adding to its height. It is as if you are demonstrating to the nefesh of the departed that during his lifetime he made an impression on you to fix up middos, to learn Torah, not to be selfish, to have more emunah, patience and selflessness etc. In other words the visitor to the grave is conveying that the departed's legacy will be continued by you and thereby will cause that his spiritual levels of reward will become higher as symbolically portrayed by your placing a stone on top of his מצבה as if just now it became extended higher.
The weight of that pebble carries a much heavier load than we think. For both you and the niftar will derive continuous benefit and reward from it.