I guess it's a similar reason for why nations sponsor cultural exhibits, film festivals, and things of that nature in other countries: it is felt that they build awareness of, interest in, and goodwill toward their country. That may translate into more tourism, but I suspect it has more to do with translating into more investment and a more receptive environment to the promotion of their interests by lawmakers in another part of their world.
It's hard to know whether Epcot really has a big enough effect to be worth the investment. I would suspect for a country like Norway it may well be a decent investment because the country normally doesn't loom all that large in the American imagination and so exposing a few million people a year to its culture as one of a select group of nations at a major tourist destination probably has a decent impact on its profile in America. For a big country like the UK, France or Germany it would probably be harder to justify the expenses as there are likely already plenty of outlets where people are exposed to their culture.
I remember reading about some Korean businessmen who kept proposing to fund a Korea pavillion at Epcot a few years back but Disney knocked them back because they didn't think it would be appealing enough to guests, so who knows how many pavillion proposals Disney has refused over the years.
