Expropriation combines the effects of Colonization and Land Surveyors, both of which are rendered obsolete by it. It's useful if you plan on making a late-game push to expand your empire or buy tiles around your existing cities (perhaps to claim a hard-to-find resource or establish a National Park). However, this is not exactly the most useful card just because of when it becomes available: in the Industrial Era, you usually don't train many Settlers and have a strong enough economy to afford the tiles you need.
Expropriation – as opposed to eminent domain, which usually involves some form of compensation – is the government taking private property for a purpose deemed to be in “the public interest.” Often this is land, but in the 20th Century it became a major element of nationalization, as governments sought to redress perceived wrongs or stabilize their economies or lessen foreign influence (or all three, as in the case of the Mexican expropriation of American oil company holdings in March 1938). Communist policies in the 1920s and across Eastern Europe after World War II saw the expropriation of virtually all foreign holdings in those countries.