Exactly this if I would not track it in my Calendar I would forget the birthday of my grandparents who I love dearly. It just cannot compute in the web of information that is in my brain. I need a crutch for it.
And there are events less personably important to me that matter to others, like “tell your son happy anniversary!” I barely know their kid, but it makes the person hearing it happy that someone thought of a nice happening in their life. Now their day is slightly brighter. Darned if I’d ever remember that without something to nudge me.