In the tractate of Gemara Berachot, on page 10b, the discussion focuses on a commentary by Tosfot regarding the opening words, "as it is said of them, glory." The text establishes that a mourner is obligated in all the commandments of the Torah with the specific exception of ritual phylacteries. This exception exists because the ritual phylacteries are described by the term "glory."
In the work Perush Rashi, a specific explanation is provided for why this term results in an exemption. Rashi explains that a mourner is in a state where his pride is brought low, which is expressed by the placement of ashes upon the head in a moment of deep distress. These ashes are placed on the crown of the head, the exact location where the ritual phylacteries are meant to be worn. Rashi concludes that it would be improper to place an object of glory in the same location as ashes. This interpretation explains the language of the discussion, linking the exemption to the physical state of the mourner.
However, an analytical challenge is raised: why is this specific reasoning regarding ashes and glory necessary? One could instead derive this law from the biblical account of the prophet Yechezkel. As recorded in the tractate of Mo'ed Katan, God informed the prophet of his wife's impending passing and commanded him to continue wearing his glory—his ritual phylacteries—upon his head. The implication of this unique command to the prophet is that, for any other mourner, wearing ritual phylacteries is forbidden. If the law can be derived directly from the instructions given to the prophet, the reason concerning the placement of ashes seems superfluous.
To resolve this, the Sar MiKotzi explains that the reasoning provided by the commentary is essential to prevent a logical error. Without the specific explanation regarding the physical conflict between glory and ashes, one might apply an a fortiori argument to other areas of law. A person might reason that if a mourner is exempt from a commandment as significant as ritual phylacteries, he should certainly be exempt from all other commandments. By providing a reason that is unique to the physical placement of the ritual phylacteries, the law makes it clear that the mourner remains obligated in all other religious duties.
Alternatively, the Sar MiKotzi suggests that this reasoning is necessary to define the duration of the exemption. The state of having ashes on the head is specifically associated with the first day of mourning, which is the period of greatest intensity. If the law were derived solely from the example of the prophet, one might assume the prohibition against wearing ritual phylacteries lasts for the entire duration of the mourning period. By anchoring the law in the presence of ashes, it teaches us that the exemption applies strictly to the first day, and the mourner becomes obligated to wear them on the subsequent days.