Shabbos isn’t something we have to do so much as it’s something we get to do! We honor God and Shabbos by dressing up. This also applies during the week (though to a lesser extent since Shabbos must be different) because we need to “represent” for God. A Torah scholar is trusted to claim a lost object even if he can’t remember an identifying mark. Depending on the depth of his learning, he might also be trusted to handle a community responsibility, lead the community, or even lead the Yeshiva. Dressing in a slovenly fashion may indicate he’s not qualified for such responsibilities. In ancient times Shabbos was announced with shofar blasts, even if a Festival fell on Friday, but not if Yom Kippur fell on Friday. Even Yom Kippur, however, is not as holy as Shabbos. If a Festival falls on Sunday, we recite a special Havdalah on Saturday night to separate between the higher holiness of Shabbos and the lesser holiness of the Festival.