For foreign companies:
Discount brokers usually rip off customers on foreign currency exchange rates, and foreign exchanges have high transaction costs compared to American ones.
Maybe this helps too:
Esprit was founded as an American company, derives most of it's profits from European markets, and is listed on the Hong Kong Exchange. Buying shares would be investing in a retail clothing business in Europe(Euro), that happens to be listed on the Hong Kong Exchange. If they convert profits into Hong Kong dollars then the cash they hold is pegged to the USD.
Yum! Brands is an American business that operates KFC, Taco Bell, and Pizza Hut listed on American exchanges. 48% of their revenue comes from America, and 52% comes from foreign markets.
Millicom International Cellular started as an American company, is now listed on American pink sheets, with revenues from operations in Latin America and Africa.
Many foreign countries have their telecoms listed on American markets. China Mobile is the state owned telecom operator in China, listed on the NYSE as an ADR. All their revenues are in Chinese Yuan.
You are purchasing a slice of a business operation!
Hope that helps.