Speaking in um,uh and er

Why do we use um,uh er and other fillers while speaking?

I um.. tend to use them when er.. I am doubtful of how those listening will comprehend uh!

"up to 8 percent of the average person’s word output consists of
meaningless fillers and placeholders like um, uh and er. "

Books of The Times: Uh, Lead My Rips: No More Bloopers