So they can know who they are talking to. IDK in the US, in Spain most services use DNI (national identity document 00000000A) to identify a client and the telephone number as most easy way to know which location is the customer asking info about (people might have more than one landline, DSL or whatever service).

Spanish law also require a second ID verification to disclose some information (let's say your wifi password), in our case it's the las 4 digits of the bank account number.

Also to pipeline you into different call centers, because it won't be reasonable to demand agents to know all the tech and commercial info about the company, it's just too much.