If you're not joking, I think the reasons are pretty clear. As a cis straight man, it's a useful way to signal your partner's importance to you. It's an opportunity for your partner to make herself look good and get dressed up which improves her mood and well being. It's also a good way to avoid letting all your time being doing your own thing or sitting around on your phones and make it clear that you aren't taking your partner or the relationship for granted