In Ashkenazic synagogues (including Reform and Conservative congregations) and some Sephardic congregations, the Sefer Torah (Torah scroll) is topped with a crown.  The crown is removed when the Torah is to be opened for reading, then replaced when the Torah is re-closed afterwards.
While I came to understand the relationship between the other “garments” of the Sefer Torah — the choshen (breastplate), the rimonim (bells), the me’il (covering), etc. — and their association with the garments worn by the kohanim/priests, I had taken the crown as being only an ornament. At Shabbat services yesterday, however, I realized that it represents the pervasive Jewish theme of “God is King.”
This theme, prominent and explicit on Rosh Ha-Shanah, is otherwise present in our daily lives, too. God as “King” means that there is only One Power over everything that happens in, to and around us. It is the essence of Torah and, in a wider sense, of Judaism itself. The “Shema” has this literal meaning, too.  In fact, during the “Malchuyot” (“Kingship”) section of the Amidah on Rosh Ha-Shanah, the “Shema” counts as one of the ten declarations of God’s Dominion. As the “Shema” appears in the mezuzah and in the tefillin, it further emphasizes the centrality of this theme in our daily lives.
So, the crown on the Sefer Torah is another reminder: “God is King.” God is in charge of every detail and activity that occurs within Creation.
That is certainly a central theme of Torah itself. It shouldn’t then be surprising that the same theme is represented in the ornamentation of a Sefer Torah.
But is Torah to be taken as “King?”
Wouldn’t that be idolatry?
It helps here to remember that in Kabbalah (and, to some extent in Hasidut), Torah is “the Name of God.” The essence of Torah is Divine; it is God’s own Essence.
“…the Torah is not outside of God and He is not outside of the Torah, and that is why the sages of the Kabbalah were justified in saying that the Holy One, Blessed be He, is Himself the Torah.” 
To “participate in Torah” is to “participate in God’s Own Essence.” To the extent that we participate, whether by hearing or reading or learning or doing, we “participate (as it were) in God, as well.
Some tzadikim have even been described as being a “mezuzah” — an outward physical case containing the Divine Essence.
Thus, it’s not the physical Sefer Torah that is God or King in some idolatrous sense. It is the Divine Essence — God Himself or Itself — that is the undiluted essence of Torah.
The “keter” — crown — on the Sefer Torah is there to remind us of it.
It remains only for us to realize it in our own personal lives.
The more I conform my thinking to the principle — the fact — of God’s Kingship, the more that the symbols I see in synagogue reverberate with that meaning.
Everything seen in a synagogue can have spiritual significance — even the decorative elements.
“The earliest Torah ornaments are the Torah crown and the finials mounted on the Torah case or on the staves of the Torah scroll. We first hear of a Torah crown in the 11th century, in a responsum of *Hai Gaon concerning the use of a crown for a Torah scroll on *Simḥat Torah. The use of the Torah crown is linked in this responsum to the custom of crowning the so-called “*Bridegrooms of the Law,” i.e., the persons called up on Simḥat Torah to complete the annual cycle of the Torah reading and to initiate the new cycle. At the time, the Torah crown was an ad hoc object made from various decorative items, such as plants and jewelry. About a hundred years later, fixed crowns, made of silver and used regularly to decorate Torah scrolls in the synagogue, are mentioned in a document from the Cairo *Genizah. Their earliest depiction is in the 14th-century Spanish Sarajevo Haggadah.
Torah crowns are used in almost all communities (the exceptions are Morocco and Yemen), their design being influenced in each locality by local tradition. The onion-shaped or conical crown of the Iraqi-Persian Torah case follows the tradition of the crowns of the Sassanid kings, the last Persian dynasty prior to the Muslim conquest. In Cochin, India, and in Aden, the independent port of Yemen, a tapering dome-like crown developed through which protrude finials mounted on the staves on which the Torah scroll is wound; the crown is not fixed to the case. By the 20th century, the Torah crown in Cochin showed distinct European features. In Eastern Iran, where the Torah had a small crown, the outer sides of the crown lost their spherical shape and became flat dedicatory plaques. Today this crown looks like a pair of flat finials, and only their designation as “crowns” hints at their origin in the Torah crown. The circlet or coronet on the Mediterranean case, which became an integral part of the case, was based on a local medieval crown tradition typified by floral patterns. The European crown is shaped like a floral coronet with arms closing over it. In Eastern Europe a two- or three-tiered crown developed, inspired by the crown motif on the Torah Ark in this region. In Italy, on the other hand, the Torah crown was a coronet, known in Hebrew as the atarah.”
 The Tanya, following the Zohar, says that the “Baruch Shem Kavod…” signifies the “lower Unity” — i.e. that God reigns over every detail of Creation, while the “Shema…” signifies the “Upper Unity” — nothing exists other than God.
 Scholem, G.; On the Kabbalah and Its Symbolism; p. 124;
quoting Recanati, Rabbi Menahem b. Benjamin; Italy, 13th-14th c.; Ta’ame Ha-Mitzvoth; p. 3a)
Note: This is why we say “B’rich Sh’mei” when removing the Sefer Torah from the Aron.