It’s so they have a space for their wheeled luggage bag in the overhead space. As more seats with less leg room get crammed into the plane and emergency equipment no longer fits under the last row of those seats there isn’t enough space for everyone to toss a wheels bag up there anymore.
The most efficient way to board a plane is to have the first person on go all the way to the back row and take a window, next the middle next to him, next the aisle, etc etc till the first row is full. However seats that are more comfortable are in the front and wealthy people / business travelers who work for wealthy companies want to sit up front, they want to be in an aisle seat; and they want their bags to be able to fit in the overhead. The airlines created boarding groups in order to be able to charge people for that combination of luxuries.
Funnily enough the last people who get their bags on often have to place it rows behind where they are sitting. It’s still faster than going to baggage claim, but it’s annoying to have to wait for 10 rows behind you to get off in order to go back and grab your bag.