On naming the "To:" person on gifts. . .

Also some people with close birthdays will celebrate together. The "to: X" becomes useful.

Also in many gift giving situations multiple people are receiving gifts at once leading proper identifiers being necessary. This leads it to just be norm even when at events like b-days and weddings when it is not strictly necessary.

Also lots of people get a bit of a high on seeing their name on a gift. It reinforces the dopamine rush of "mine".

And finally. It is dang useful if you have multiple presents laying around prepping to off to various recipients. Makes sure you don't send an adult themed wedding shower gift to your nine year old niece's birthday (or other such embarrassments).