Maybe you want to understand when foreigners start talking, then the first few syllables are usually not understandable. This is a natural thing. The hearing needs to adapt to the unknown voice, and then the understanding can start.

So if somebody yells at me from behind “xxx yyy left” I would first slow down a little (because yelling creates some feeling of alarm, and slowing down is simply the automatic way to increase safety) and then I would look to my left in order to find out what that person might have been trying to tell me.

A greeting formula like “Hello”, or whatever is common in your region, is helpful for this acoustic adaption. And of course it is the way to start any communication in a friendly and respectful way. Leave it away and you are rude, automatically.

If you don’t care being rude (like some of the comments seem to suggest), it is still a good idea to always speak a few more syllables. The last ones will be understood better.