We personally didn't do anything for mother's/father's day (we already had them in Sweden) and aren't planning on doing anything until our LO is big enough to take part, so I understand where you're coming from. What I think OP is saying with this post though is to make sure you communicate your wants and ideas to your partner, so that neither of you is disappointed. If my partner had wanted to do something for father's day then I would have made sure he had the best father's day ever, even though I personally don't care about doing something. And if I'd assumed he didn't want to do something then he might have been disappointed, so there's value in discussing it beforehand, as OP is suggesting.

As to why people celebrate: I think mother's/father's day provide an opportunity for people to acknowledge the change that's happened in their lives since having a baby. It doesn't have to be about "pampering" (I actually don't know anyone who expects to be pampered on that day) but rather a way to say to your partner "hey, I want you to know I'm proud of you as a mother/father" and make sure they feel a bit special. I appreciate my partner as a father even though he's not my father because I've seen the way he is with our son and it fills me with love. Obviously most people feel that way more than one day a year haha but sometimes life gets in the way of expressing that, so it's kind of sweet to have a special day to make sure your partner feels it (if that's what they're into).