It's just a discourse marker that has become a common verbal quirk. Sometimes it indicates a qualification or correction, sometimes it signals a speaker's uncertainty or lack of confidence, but other times it's just a meaningless filler phrase. Like "like" or "well".

I mean, people use all sorts of verbal tics and filler phrases without realizing it. Chances are you probably have a few of your own that you don't realize. I agree that it can be annoying and hard to stop noticing once you notice it, but it's ultimately unproductive to get too hung up on it. And I say this as somebody who used to keep a secret tally of the number of times my classmates said "like" or "uhh" in class.