Well, It's not mainly to show off for sure... It's basically used to play arpeggios with more speed... Anyway, you can play anything with high bpm to show off.
The most important thing to keep in mind while practicing sweeping (as with other techniques) is not to be in a rush to play fast , speed comes by time.
You seem like a Metallica fan (like me ;) ) , so I'll tell you where you can find easy sweeps in their songs to begin practicing.
1- Lepper Messiah Solo ... The begining of the solo is just 3-string sweeing throughout ... You should master this one as a first step, cuz you'll find these min/maj 3-string sweep a lot in other songs.
2-Creeping Death Solo ... There's a minor 3-string sweep there , in the middle of the solo as I can remember.
3-Fade to Black intro solo ... Some people play this one with alternate picking ... Anyway I prefere sweeping'em ... You should try sweeping too , at least for practicing... (what doesn't kill you makes you stronger).
4-The Call of Ktulu Solo ... There's a realy cool 4-string sweep in the middle of that solo , it's hard but worth practicing.
You should try some Malmsteen too ... You'll find the 3-string dim sweep in all of his songs !
Marty Friedman makes realy cool & complicated sweeps , Megadeth's "Rust In eace" should be your next step.
And for sure we can't forget the King of sweeps , Jason Becker.