If you live in a crowded city, are in a crowded shopping mall or other event where many people are walking both directions, you don't have to move out of stranger's ways when the walk in your path. I have found that greeting them does one of a couple of things.
They may greet you back. (this is least common but common enough for a quick hello and many conversations have begun here.)
They may just smile
If you pass them again, they are much more likely to remember you. (I have people come up to me or start the greeting second time around.
and lastly, if they are not in a social mood, they move out of your way pleasantly, returning your courtesy.