I agree with Victoria. "You know" or "like" in every sentence can get annoying, but we are all guilty of the odd hmmm...., or um...

It gives some thinking time if you need to work out how to respond, and few people manage to avoid the occasional "filler". But it can be very annoying and distracting if someone says "you know" at the end of every sentence.

Most people rarely use fillers on purpose. Mostly they just happen and we are not aware of them until we hear a recording of ourselves. Then we try to cut them out.

As Beth and Victoria say, the occasional "hmmm" or "mmm" in conversation will go unnoticed, or may even be encouraging to the other person to show you are listening. If you are making a presentation or video though, or any type of solo speech, they are not good.