Why do you think that joule's father built him a science lab when he was young?

While I don't know the story, I would hazard a guess that he wanted to get his son interested in science.

James Prescott Joule (1818-1889) was the son of a well to do British brewer. He was homeschooled for health reasons. The family was wealthy and one of his tutors was John Dalton, a pioneer in atomic theory. He and his brother experimented with electricity during their childhoods and took over the running of the brewery in their mid-teens. During the years he was running the brewery, he experimented with different sources of power to run the brewery. As an adult he was a physicist and brewer, famous for atomic theory, studies on color blindness and the development of the measurement of energy, in units called Joules, in collaboration with Lord Kelvin.