Of course you could make 1 line and zigzag all the way through your city but people wont use the line because walking from their home to their destination is faster than zigzagging with your bus through the whole city. More lines makes it possible for people at B and C to get to A in time :).
Also, with this multiline configuration, people can still get from C to somewhere between the crossing of both lines and B. (leisure stuff).
The person that takes the purple route with 2 lines that cover a city:
The person that takes the purple route with 1 line that cover a city: