I've got to agree here with the later posters.
Yes, as Matt says, we can't always articulate perfectly and a simple 'yeah' or 'yes' can often reinforce a thought.
There's a lot of it in the blues, admittedly often as a substitute for repeating the first line, but can sound great.
And what about all those soul singers....James Brown, Al Green, Mmmmm-Marvin Gaye...who punctuate their performances with some lovely 'filler'.
And, in fact, on its own cement isn't as strong without an appropriate amount of sand....