It wasn't. The British empire wanted money, land, and soldiers among other things

Examples of each:

Money:

South Africa was taken for trade. A coaling station was set up there and when diamonds and other valuable ores were found they just continued to take all of South Africa and continued up the east coast. The Suez canal in Egypt also was very profitable.

Land:

It was a land grab during the imperialist times of the 19th century. France, Germany, USA, Belgium, Russia, Japan, China, and Italy, among others, were taking giant swaths of Africa, Asia, and the Pacific Islands. Britain was grabbing all they could.

Soldiers

While maybe not a reason for their imperialist goals, it was certainly a happy side effect. Soldiers from India, New Zealand, and Australia all fought in WWI and WWII.