金剛經纂要刊定記 3.32

3.32

示現下。釋通也。此有三意。一、順世故，夫人外歸必恐塵染，故須洗足，佛順亦爾也。二、表法故，洗去煩惱垢染，顯得清淨法身也。三、為後軌者，如資緣說。

The passage beginning "Manifesting..." explains and resolves it. This has three meanings: First, to accord with worldly ways, for when an ordinary person returns from outside, there is always fear of dust and defilement, so they must wash their feet; the Buddha's acceding is also like this. Second, to express the Dharma, washing away the defilements of afflictions to reveal the attainment of the pure dharmakāya. Third, to serve as a model for posterity, as explained under the requisites.