Why would you get a more accurate measure of the measure volume of an irregular solid by using displacement instead of measuring with a ruler?
I think you should tell us the answer to this.
And we don't even need to use an irregularly shaped solid, we could compare the methods with a sphere. Unless I had a pair of calipers, I could not measure the diameter of the sphere. With an irregular solid I have no way of determining its volume, except by the displacement of water, the which would give me a reasonable estimate.