Why do the laborers who moved Dracula’s earth said that the job was “dry work”? And why did they prefered a “liquid form”?

What they preferred was a "liquid form" of appreciation...... meaning alcohol.

Their tally was exact with the list, and they had nothing to add to the simple description `fifty cases of common earth', except that the boxes were `main and mortal heavy', and that shifting them was dry work. One of them added that it was hard lines that there wasn't any gentleman `such like as like yourself, squire', to show some sort of appreciation of their efforts in a liquid form. Another put in a rider that the thirst then generated was such that even the time which had elapsed had not completely allayed it. Needless to add, I took care before leaving to lift, forever and adequately, this source of reproach.