They are definitely not useless. It's one form of rewarding the customer loyalty with exact zero costs for an airline.

Besides that people like to share space (and the travel environment) if possible with fellows belonging to the same social class, hobbies, income group or whatever criterion you have.

Many business people just feel more comfortable when seated to another business person instead of one of the 119er return ticket holders without any status. So just a special type of preference respectively self-cognition.

If you additionally try to place a "last seat out" on the forward rows of Economy you could try to keep the middle seat free. Which is indeed diminishing if the seat load factors go beyond 60 %.

On very short hops there is additionally the chance to enjoy the light snack and beverage slightly longer. Just think of a one aisle short/haul aircraft where service startes from the first rows and the last ones simultaneousely.

If you are in rows 15-20 you get your service latest. And on the one hour hop have nearly no time to enjoy the fresh-brewed coffee. So the preferred seats are a tiny gesture to loyal customers and have some soft product advantages. ^

On some airlines like BA which have a convertible Business/Economy layout with a higher seat pitch in front these preferred rows are a chance to get some valuable inches of legroom compared to the non-preferred seats.