For me (and everyone's personal mileage will vary), one reason is that marriage is a big milestone, and it's good to have formal celebrations to fix these milestones both for ourselves and the other people in our lives.
Other people's milestones and how they celebrate them are clearly different than mine - I like weddings, but graduations bore me to tears (including mine). But I know people for whom walking was a direct affirmation of the amount of time and work they spent on their degrees.
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Other people's milestones and how they celebrate them are clearly different than mine - I like weddings, but graduations bore me to tears (including mine). But I know people for whom walking was a direct affirmation of the amount of time and work they spent on their degrees.