You need to inform your employer of your pregnancy or your partner's pregnancy for a couple of reasons. If you are the one who is pregnant, informing your employer will legally protect you from discrimination related to your pregnancy. Should your pregnancy affect your work in any way, under the law, you can't be discriminated against and could seek assistance in the workplace because of it.
Regardless of whether or not you're pregnant or if your partner or you need to take parental leave for another reason — like if you're adopting a baby — your employer needs to be made aware that you'll be out of the workplace, so they can cover your work accordingly. While you don't need to overshare details with your boss, it's important they know the facts, like when you'll be out, so they can support you and your job in the workplace.