Two-part security codes

By e-mail or snailmail I occasionally receive a letter that tells me two pieces of information that I need to type into something else, e.g., for voting via a website.

I don't know what the point is of dividing the code into separate components. From a security point of view it seems to be just one code: they send me the components together and I use them together.

Is there typically some other use case that I am not seeing, in which the components become usefully separated? Are the organizers just trying to make the security look better than it actually is?