The tone or timbre of the open string note and the fifth fret note are a bit different. I'll use either, depending on the sound I'm after. Sometimes, I'll play a riff or even an entire song on a single string because the notes played on that string have the timbre I want, even though I could play some of the notes on other strings. Also, it is easier (IMO) to mute a string down the neck (e.g. 5th fret) than up at the nut/first fret. So you have better control and less bleed through of your notes.
