It's not a universal rule.
A former boss of mine used to do it a lot and explained the habit by saying that if you say "yeah" all the time, you imbue your listeners with positivity, so they're more likely to say yes when you ask them a question.

Then again, he learned the technique from his boss, who *was* a shouty useless cunt.
( .Yeti., Thu 10 Jun 2010, 13:35, Reply)