It is a pledge to a person rather than merely a promise to do something. A Christian bridegroom understands that he could not have the privilege of living with his bride if he would not make a marriage vow.
فَنَحْنُ ،‏ رَغْمَ نَقْصِنَا ،‏ نَرْغَبُ فِي أَنْ نَكُونَ مِلْكًا لَهُ وَقَدْ عَقَدْنَا ٱلْعَزْمَ عَلَى ٱلْبَقَاءِ أَوْلِيَاءَ لَهُ مَهْمَا حَدَثَ .‏ —‏ مت ٢٢ :‏ ٣٧‏ .‏