The commandment of Orlah was instituted for its spiritual benefits. This approach subdivides, with some focusing on how it serves to distance the nation from idolatrous rites and others on how it brings one to recognize Hashem.
Distancing from Idolatry
The prohibition to eat of the fruit of a tree in its first three years is meant to keep the nation from imitating idolatrous practices.
Idolator's customs – The Rambam suggests that people in surrounding nations would engage in various magical rites to accelerate the pace at which their trees would bear fruit. When the fruit appeared they would then bring of it to the gods in whose name the magical rites had been performed. To prevent people from imitating these rites, Hashem prohibited benefiting from any fruit grown in the tree's first three years, ensuring that there would be no need for anyone to try and hasten the fruit's production.
Connection to נטע רבעי – This commandment, too, constitutes a reaction to these foreign practices. In contrast to the surrounding cultures who brought of the fruit to their idols, Hashem mandates that we bring the fourth year's fruit to Hashem.
Meaning of "ערלה" – The word "ערלה" elsewhere in Tanakh refers to a foreskin, and the word ערלים is used to describe those who are uncircumcised and not part of the Jewish faith. As such, it is possible that by choosing specifically this root in forbidding the fruit, the Torah is subtly reminding the people not to engage in foreign, idolatrous rites. They need to distance themselves from that which is "ערלה", from that which is markedly not-Jewish.
Context – This understanding of the law might be supported by the fact that the prohibitions which follow this one similarly relate to magical and idolatrous practices, banning divination and necromancy.1
Biblical Parallels – The Rambam is consistent in viewing many laws throughout Torah as being aimed at distancing the nation from idolatrous customs.
The blessing: "לְהוֹסִיף לָכֶם תְּבוּאָתוֹ" – Hashem's promise of extra produce might serve as an incentive to keep the obligation and counter the desire to hasten a tree's production. Hashem promises that if one does not attempt to have the tree bear fruit prematurely, Hashem will ensure that more fruit will be produced later. As such, there is nothing to be lost by heeding the directive, only what to be gained.
Recognition of Hashem
The commandment is intended to promote recognition of Hashem as the true owner of all, and to thank Him for His blessings.
Why is the fruit off-limits? These sources suggest that the fruit is off-limits because one cannot eat of one's fruit until it has first been given to Hashem in recognition of His role in providing them. Yet, the first three year's worth of fruit tend to be of poor quality, not worthy of a Divine gift. As such one can only bring of the fourth year's yield to God and any fruit grown beforehand is banned from human benefit.
Connection to נטע רבעי – This approach views the commandment of ערלה as being secondary to the mitzvah of נטע רבעי, only mandated to ensure proper observance of the latter.2
Biblical parallels – The two laws are similar to the laws of first-born animals (בכור) and fruits (בכורים) which are all consecrated to Hashem in recognition that all belongs to Him and that any good one has, is bestowed upon him by God. Before partaking of any "first fruits" it is necessary to first thank Hashem.3
Context – The placement of the mitzvah within a list of laws regarding holiness is expected, as these first fruits are consecrated to Hashem, and the fourth year's fruit is even referred to as "קֹדֶשׁ הִלּוּלִים לַי"י". It is not clear, however, why it follows a law of a forbidden sexual union and precedes the prohibition against "eating on blood" specifically.4
Meaning of ערלה – Ramban, following RashiVayikra 19:23-25About R. Shelomo Yitzchaki, assumes that the root "ערל" means "sealed" or "closed". Ramban suggests that since Tanakh uses the root "פתח" to describe a fruit's first appearance on a tree,5 when Hashem wanted to forbid these fruit he uses the opposite term, "closed." The first three years worth of fruit are as if closed, as if the tree had not born them.
Health Benefits
The law serves to preserve the health of the Children of Israel. Since the fruit that a tree bears in its first three years is detrimental to one's health, Hashem prohibited its consumption.
Connection to נטע רבעי – According to this position the two mitzvot have different reasons and are only connected in that both dictate what to do with the initial fruit born by a tree. "נטע רבעי" is similar to the mitzvah of בכורים, the first fruits which are dedicated to Hashem in recognition of Him as Creator.
Meaning of "ערלה" – This position might suggest that the verse intentionally uses the root "ערל" (rather than a synonym) since the word has a negative connotation in Tanakh, thereby implying that these fruit are inherently negative at this stage. [See Ibn EzraVayikra 19:23-25About R. Avraham ibn Ezra who writes, "שהוא הפרי חשוב כערלה, שהיא מזקת ולא תועיל".]
Biblical parallels – Ramban compares the mitzvah to the laws of kashrut, which he similarly suggests were instituted for health reasons.
Context of holiness –
Since holiness implies separation; any law which requires one to separate from something might relate to "holiness". As such, these laws, even if instituted for purely practical purposes, fit nicely into the chapter.
Can laws be utilitarian in nature? One might question whether the purpose of Torah is not to instill good character and deeds rather than to proffer medical advice.6 Ramban might argue that the assumption that laws cannot be utilitarian in nature is simply wrong. In fact, multiple laws have been understood by varying commentators to be instituted for purely practical reasons. See, for instance, Rambam's understanding of Shemittah and the incense altar, and Ralbag's understanding of the laws of Tzara'at.
Meaning of ערלה – R. Hirsch suggests that the root always refers to something whose usage is limited in some way, when one is prevented from making use of an object in its normal fashion. Thus, for example, one who is "עֲרַל שְׂפָתָיִם" has limited usage of his tongue.7 The fruit of our verse is so termed because one must limit one;s usage of the tree.