The lining is transparent (and usually gives the metal a yellowish hue).

Here's a demo with a Coca Cola can: https://www.youtube.com/watch?v=uQHFQoFoxvQ

Basically, you don't want any funny reactions with the metal.

Acidic foods are not good for metal. Also, there's the possibility of metallic taste being transferred to the food so cans are often lined with plastic.

Literally every can is plastic lined. They used to use wax on them, but plastic was cheaper so...