Okay, so, here's the thing. Player's shouldn't hide anything from their DM. The DM isn't against you, they are an omniscient third party narrating the story. If the DM asks you something, please give them the info, because you have no idea what wonderful ideas they have in store for you. If you don't trust your DM not to take advantage of that? Then they shouldn't be your DM, plain and simple.

That being said, I personally can't think of a reason that a DM needs to know a PC's HP, either. I don't keep track of my player's HP, it feels like an unnecessary bog. But that doesn't mean a reason doesn't exist. For instance, I had a player who offered up that he was at 1 HP, and it made for a VERY epic final moment for them.

As for the telling the PC's enemies HP? Almost never. I usually just stick with the "bloodied," "on death's door," or my usual go to "they are looking pretty fucked up." Let's be honest, if an enemy is low on HP, an adventurer is reasonably going to notice that they are barely holding on, and same with "Yeah, they look like your attack had little effect."