A study by Mark Liberman on the use of um and uh in spoken English looks at use of these filler words – and the frequency – by gender.
We use them in the act of speaking, when we need a moment to clarify our next sentence. In (unscripted) conversation, silence is an invite for others to fill the space.
Um and uh signify we’re not finished.
Quirky note – males use uh 14% less when talking to females, who in turn increase their uhs when talking to males.
From a communications viewpoint, it looks like a subconscious case of knowing and accommodating your audience.