It usually comes down to separation. Although some airways are procedurally separated (albeit radar monitored) many aren't, so even though if you all maintained the flight plan route there would be no loss, we can't take that chance, so often have to assign hdgs - I usually don't need to know what they are, just that you stay on them.
Other examples include: being close to the edge of a sector and an adjacent sector has traffic close to their edge too - you will never hear the aircraft on frequency but we still need to ensure separation;
The next controller ask us to lock you on your heading to ensure you stream behind other traffic that could be a hundred miles away at that time, and coming from a completely different direction;
HTH