No nation is supposed to place a visa sticker on top of other nation's stamps. There are a great many different uses that passport stamps get used for down the line, some of which have already been mentioned. They can be used to document arrivals and departures should the immigration computer records be incomplete for whatever reason. Some nations stamp passports when you visit their embassies, so they can detect "embassy shopping" for visas and the like. Some nations stamp visa application refusals for the same reason. A deportation order might be stamped in a passport, and that country relies on it remaining in the passport if they don't have adequate computer systems. For tax reasons, you may want to be able to establish that you were outside your own country for more than 50% of a particular year. There are too many other reasons to list here.