The original idea was to use existing telephone lines for high speed data communication. DSL and telephone service can coexist on the same phone line and do not interfere with each other. From the Telco's point of view, DSL gives them the ability to sell the same telephone line twice, to the same customer. It is an idea that benefits the telco (who makes more money) and the customer (who gets a high speed data connection without the need for installation of expensive new cables.)