Why Imperialize?

In Parsons, The Rule of Empires, he mentions the value that the imperial system brings to states that use it, with Britain as an example. Parson mentions that "Britons at the turn of the twentieth century put 75 percent of their capital in non imperial territories, and on average these investments brought approximately 1.58 percent higher returns than imperial ones." (Pg. 297) Just from these numbers a clear case can be made that it is more profitable to invest in ones own country than it is to go out and imperialize other territories. But the statistics do not stop there. This number is actually skewered because of the British control of India, which was an extremely rich investment. Of all of the imperial investments, India was the one that truly made sense. The other major colonial possession of Britain in the early twentieth century was Africa. The entire continent of Africa south of the Sahara, took in only 5 percent of all British exports (Pg. 297). Clearly Africa was not a profitable venture, and as Parson explains it was more an attempt at maintain their global empire than to bring in profit. But even so, how is taking on large swaths of nonprofitable land, to protect another imperial possessions really worth it? Would it not have been worth it for Britain to have put money into their own industry rather than spending it on imperialism, or if they couldn't go without imperialism, why not spend all that money that they did in Africa, and instead spend it in India?