When you’re in a call and you want to talk to someone or a couple people, but you’re in a group with someone you don’t particularly care for. So you have to sit there and listen to them overtalk and maybe you don’t even like their voice. But you can’t mute them or be rude so you have to waste time until they finally leave.
In an impromptu conversation this doesn’t bother me as much, but if you’re running a meeting or giving a prepared briefing, don’t say uh or uhm all the time. One, because you should know the material you’re presenting, but mainly because I’m easily distracted and will just start counting how many times you say uh instead of paying attention to the actual information.