So let's sketch a hypothetical. The party leveled up two weeks ago and Joe's PC got a new ability that's a little specialized and hasn't come up in play yet. Then the situation where it applies arrives and Joe doesn't recall he's got an answer for it, but I do. What do I do?
If I'm a player at the table, you can better believe I'll say to Joe "Hey, didn't you get a new ability to use for this?" We're friends, I want Joe to get his moment to shine, and forgetting about a new and unused ability can happen to any of us. Saying nothing would be a dick move.
Now with that established let's turn it around. If I'm the DM instead of a fellow player, do I have a reason NOT to say that to Joe? Well... maybe I do. As the DM I'm supposed to pose challenges to the players and let them come up with the solutions. If I offer up the solutions to them I'm cheating them out of the fun of the game. So maybe I don't offer the reminder immediately. If they do something else right away, fine. If they don't, then I can ask Joe for a DC 5 Int check to win a reminder that he just learned a new trick. Making him roll for it means it feels less like the DM coaching the players and more like the character recovering from a moment of forgetfulness. And of course, as others have said, if Joe's a new player he gets a lot more free reminders about how the game works.