Strangers popping up in fiction immediately alert me to the fact that this stranger has a crucial role to play or will reveal more about the hero/heroine at some point (if only because they're about to try and kill them. There will be clues as to why).
Strangers can also be used to introduce new species into fiction where this is an appropriate thing to do. Different species are always strange to the "main standard" one(s) and so strangers can be used as a contrast. They can also show up attitudes to racism, sexism and so on that are held by the main species being written about.