A director can bring a lot to how a film turns out - the two different Omen movies have virtually the same script, and they have completely different "feels" and atmospheres. The Disney remakes could be another example, although they're usually different mediums.
A director will (generally) make decisions about every aspect of the film. Like, George Lucas would give a rough description of what he wants an alien to look like, and then choose out of a couple of different designs based on that. While another director might not care too much, and leave it to the designer. Lucas was a bit of a control freak, the "average" is probably somewhere in the middle.
Big Studios usually have a very clear vision of how they want their films to look. MCU is basically a cookie cutting machine, and the Disney Star Wars movies have a very "written by committee" feel. They know exactly what they want, and they just need a director to come in and do the leg work.
Which is why I don't get why the hire directors with "visions" - how many directors have Disney Star Wars fired
s'why they tend to stick to boring studio hacks like JJ Abrahms and Joss Whedon.