I do it with my wife. For us, it's a way of:

• Learning about the other's day and what they do - whether that's work or pleasure. I think that's a big part of being in a relationship.
• If something's happened that has made one of us happy/sad, sharing that helps us support the other. It also lets them know when there's something going on that might affect our relationship. Even if they can't help, it's good to know there's a problem so they don't think it's about them when I'm unduly quiet or down.
• As someone who sometimes doesn't understand things obvious to others, it can be handy for a second opinion, or ask what they thought was meant. It also helps me post-process the day's events and square them away.

If I didn't have an SO, I'd probably do the same with my dog; although it might be a bit more one sided.