I was recently trying to figure out why a fake player I summoned would not hold an item I dropped in its hand. I eventually realized it was because I had previously used that account on my server, and I had logged off while having a different hotbar slot selected. It would be nice if there was a command such as /player <name> select [slot] so that I could make the player hold the item without having to give them dummy items to fill up some of the slots.

that would only work for fake players, but that's a valid thing to expect.

Actually, it could work with regular players too by sending them a HeldItemChangeS2CPacket (yarn).

Never understood why does it exist, but it does.

Yeah, its needed for item pickup for suvival players. Its server who controls that. Then its easy peasy