1) The reason is need to communicate verbally over the Internet or via letters.
2)English is necessary for people working in any branch of science and not only science
3) Spreading intellectual and cultural horizons. Establishing a contact between two cultures
4) Wide abroad study oportunities. GB has several summer schools for foreigners so they could improve their english. Also there are exchange visits, business and tourist trips that give wide opportunities to talk to native speakers.
5) The possibilities are infinite. International business, software localization, helping schoolchildren with their homework, reading manuals, navigating in a foreign country etc.