Vows are very highly regarded rules for the husband and wife to adhere to, to ensure that the marriage works well and does not harm either of the spouses. Vows are taken as lifelong promises, and the words of each spouse will always be held to them - something about honor and integrity that is very highly regarded in most cultures and communities.
That's because the vows are solemn promises that couples make to each other and is usually seen as rules and regulations for the married couple to abide to. These vows are usually steeped in tradition, and we all know that society has a fixation on tradition, so any variations outside of them will be considered a scandal of sort.
Wedding vows are sacred because they are indicative of the kind of relationship a couple is supposed to strive for the rest of their lives. Because a marriage is a contract between two people, and it lasts longer than most other contracts, it’s important to be clear about how the couple is supposed to proceed. Advice is handed down through generations and codified through religious practice and other customs to give the marrying couple the best chance at success. Holding the vows as sacred stresses the importance of adhering to the rules and guidelines as they’re laid out.