I'm friends with someone in college. We graduate and move to separate cities. We go from regular contact to every few months. By 2 or 3 years from graduation its just a text on birthdays. That last for a year or two. We both get married and then reconnect because of X. We become closer friends again.
That same situation is less likely to happen without the yearly contact. At the very least the friendship is maintained, even if the maintaining can only last a few years before deteriorating, and it gives the relationship a higher chance to pick back up at a later date.
And I don’t know of any meaningful relationship that is maintained just by that bare minimum.