The definition of a setdresser:

"A setdresser is the person who arranges and prepares the place where the filming is made."

In itself true but the function setdresser includes, in my opinion, much more.

The setdresser is responsible, possibly together with the art director, for many elements that come into the picture or not in the picture.
He / she is responsible for ensuring that what is being filmed is consistent with previous situations (continuity).
That the props are in the right position, in good condition.
That unwanted objects do not come into view by "dressing them away" or camouflage them.
Prepare (for example, repair, age, make more realistic or adjust) objects and/or props before they can be used in shot.
Create space on the set for the other departments (for example lighthing department, camera department , grip or sound) so that they can do their work unhindered and bring this set back to the old state.
Create the perfect beer for a pack shot, make beautiful packaging.
Making impossible things possible (hanging a lamp on the ceiling even if that ceiling is not present!)
Managing the handprops of the actors and actresses.
And much more......