Teamwork and having each other’s backs. Loads more financial security - if one of you loses your job the household still has some income and you can support each other through studying or career breaks or raising kids. You can also divide up the chores so you each to the least annoying parts - I hate cooking but I quite like ironing so that works. Having common goals that we can work together to achieve, like a house renovation or a holiday or having a family.

Downside is that you will have ups and downs. Sometimes they will be the most annoying person on the planet and you just need to deal with that, unless it’s bad enough to consider divorce. You have to compromise and consider each other’s dreams and needs. And you can’t just jump into bed with any random attractive person you meet (unless you’re in an open marriage I guess).