Sometimes when watching someone play guitar they will play some notes let's say down around the 12th fret then slide their hand all the way down the neck and back up to where they left off.  Is this just showmanship or does that really serve a function like muting the strings?  I've been meaning to ask my instructor this.
Buzz

I noticed the same thing.  My coach has a playing buddy from 40+ years ago and I asked him.  He showed me that there is a subtle difference in the sound.  It is especially noticeable with an accoustic, whether you strum OVER the sound hole, above the hole or bellow it.

Newbie

well the way i'm thinking you mean is when they do it between 2 licks quite quickly.

it kinda serves a purpose in that it's an effect but also kinda showing off too

I would think that the slide your talking about, sometimes is done to add a different intonation, or show the persons abilities,(ie), doing a slide of 8-10 frets, stopping  with out losing tempo,in the proper key.Sometimes it's done to  add emphisis to a passage. there are various purposes.

great point i never noticed that sliding up and down the neck ;but i would thank it gives the note more flavor

It adds variety and feeling to notes when you do that ... the same as pick slides, bends, vibrato, etc.

Think of it the guitar as speaking ... if you just play with no effect on your strings, it can get boring ... liven up the speaking and it sounds great