I just watched a hilarious remix of Willy Wonka saying to Charlie Bucket that he had lost and wouldn't win anything.
In it I noticed that Willy Wonka's possessions seem to be chopped in half. Half a desk, half a magnifying glass...etc.
Why is it?
To the best of my knowledge, it has no intrinsic meaning.
Wonka is eccentric and wonderful, making magical things mundane and mundane things magical. The entire factory is an example of this, and it should be no surprise that Wonka's office, despite being a more serious environment (with him doing mundane things like paying bills and sorting invoices there) still has that 'Wonka Touch'.
It is also a reference to the Tea Party in Alice's Adventures in Wonderland. Wonka himself, at least his design in the referenced movie, owes something to the Mad Hatter. In the Disney version of Alice, which was produced prior to this movie, the half-tea cups and other bisected items were a prominent feature.
A further study of the relationship between Alice and Wonka is beyond the scope of this answer, but suffice to say, Wonka's factory is very much Charlie's version of Wonderland, and the movie knows it.
As pointed out in comments on this answer by NominSim, Mel Stuart addressed this in his book Pure Imagination: The Making of Willy Wonka and the Chocolate Factory. He had this to say:
the reason everything in Willy's office is cut in half was because Stuart couldn't bear the thought of - after having gone through all the whimsical and creative rooms in the factory - ending the movie in an ordinary office. Everything was cut in half to make the room look more Wonka-esque.