So why isn’t our speech fluent? Studies suggest that we verbalize hesitations because we’ve been conditioned to fill the void even when we don’t have something to say. For example, we use “um” and “ah” to hold onto the “conversational floor” as we are planning what we are going to say next, with “ah” signaling a short delay and “um” signaling a longer delay.

https://hbr.org/2018/08/how-to-stop-saying-um-ah-and-you-know?utm_medium=email&utm_source=newsletter_weekly&utm_campaign=weeklyhotlist_not_activesubs&referral=00202&deliveryName=DM11440