There's a joke by comedian John Mulaney where he remarks on how he loves finally being married way more than simply dating. That's because now he can finally refer to his partner as "my wife" to strangers instead of as "my girlfriend", which, as he explained, just made him feel silly and juvenile.
It's a funny joke, if you actually hear it from him and don't just read it in an article, but I think Mulaney really makes an excellent point here.
There's a certain power in being able to call someone your legal husband or wife. It makes you feel like you've done something right, something mature and adult, and it makes your relationship somehow feel more...well, just more, I suppose.